Collapse and Conservation of Shark Populations in the Northwest Atlantic

Abstract

Overexploitation threatens the future of many large vertebrates. In the ocean, tunas and sea turtles are current conservation
concerns because of this intense pressure. The status of most shark species, in contrast, remains uncertain. Using the largest
data set in the Northwest Atlantic, we show rapid large declines in large coastal and oceanic shark populations. Scalloped
hammerhead, white, and thresher sharks are each estimated to have declined by over 75% in the past 15 years. Closed-area models
highlight priority areas for shark conservation, and the need to consider effort reallocation and site selection if marine
reserves are to benefit multiple threatened species.

Full text

DOI: 10.1126/science.1079777
, 389 (2003);299 Science
, et al.Julia K. Baum
Atlantic
Collapse and Conservation of Shark Populations in the Northwest
This copy is for your personal, non-commercial use only.
clicking here.colleagues, clients, or customers by
, you can order high-quality copies for yourIf you wish to distribute this article to others
here.following the guidelines
can be obtained byPermission to republish or repurpose articles or portions of articles
): December 29, 2010 www.sciencemag.org (this infomation is current as of
The following resources related to this article are available online at
http://www.sciencemag.org/content/299/5605/389.full.html
version of this article at:
including high-resolution figures, can be found in the onlineUpdated information and services,
http://www.sciencemag.org/content/suppl/2003/01/15/299.5605.389.DC1.html
can be found at: Supporting Online Material
http://www.sciencemag.org/content/299/5605/389.full.html#ref-list-1
, 5 of which can be accessed free:cites 13 articlesThis article
247 article(s) on the ISI Web of Sciencecited by This article has been
http://www.sciencemag.org/content/299/5605/389.full.html#related-urls
34 articles hosted by HighWire Press; see:cited by This article has been
http://www.sciencemag.org/cgi/collection/ecology
Ecology
subject collections:This article appears in the following
registered trademark of AAAS.
is aScience2003 by the American Association for the Advancement of Science; all rights reserved. The title
CopyrightAmerican Association for the Advancement of Science, 1200 New York Avenue NW, Washington, DC 20005.
(print ISSN 0036-8075; online ISSN 1095-9203) is published weekly, except the last week in December, by theScience
on December 29, 2010www.sciencemag.orgDownloaded from

Collapse and Conservation of
Shark Populations in the
Northwest Atlantic
Julia K. Baum,* Ransom A. Myers, Daniel G. Kehler,
Boris Worm, Shelton J. Harley, Penny A. Doherty
Overexploitation threatens the future of many large vertebrates. In the ocean,
tunas and sea turtles are current conservation concerns because of this intense
pressure. The status of most shark species, in contrast, remains uncertain. Using
the largest data set in the Northwest Atlantic, we show rapid large declines in
large coastal and oceanic shark populations. Scalloped hammerhead, white, and
thresher sharks are each estimated to have declined by over 75% in the past
15 years. Closed-area models highlight priority areas for shark conservation,
and the need to consider effort reallocation and site selection if marine reserves
are to benefit multiple threatened species.
Human exploitation has propagated across
land, coastal areas, and the ocean, transform-
ing ecosystems through the elimination of
many species, particularly large vertebrates
(1, 2). Only in the past half century, as fishing
fleets expanded rapidly in the open ocean,
have large marine predators been subject to
this intense exploitation. Many species, in-
cluding tuna, billfishes (3), and sea turtles
(4), are of immediate conservation concern as
a result. Among the species impacted by
these fisheries, sharks should be of particular
concern. Despite their known vulnerability to
overfishing (5, 6), sharks have been increas-
ingly exploited in recent decades, both as
bycatch in pelagic longline fisheries from the
1960s onward (7) and as targets in directed
fisheries that expanded rapidly in the 1980s
(8). The vast geographic scale of pelagic
marine ecosystems constrains our ability to
monitor shark populations adequately. Thus,
the effect of exploitation on sharks has, for
most populations, remained unknown (9).
Shark management and conservation have
been hindered by the lack of knowledge on
their status or even the direction of the pop-
ulation trends.
We present an analysis of logbook data
for the U.S. pelagic longline fleets targeting
swordfish and tunas in the Northwest Atlantic
(Fig. 1). Pelagic longlines are the most wide-
spread fishing gear used in the open ocean.
The data set presented is the largest available
for this region (214,234 sets between 1986
and 2000 with a mean of 550 hooks per
longline set) and includes one of the longest
time series for sharks. Six species or species
groups were recorded from 1986 onward, and
eight species from 1992 onward (Table 1).
For most shark species examined, this is the
only data set from which reliable abundance
trends can be estimated for the Northwest
Atlantic (10). It is also one of the only avail-
able sources worldwide from which the ef-
fects of exploitation on sharks in the open
ocean can be investigated. However, consid-
erable unreporting may occur in logbook
data, and missing values cannot be distin-
guished from true zeros (11). To address this
problem, we developed a method to model
the positive catches using generalized linear
models (GLMs) with a zero-truncated nega-
tive binomial distribution (12, 13). Our meth-
od assumes only that if a positive number of
sharks is recorded for a set, then it is approx-
imately correct. We standardized catch per
unit effort (CPUE) time series for area, sea-
son, fishery variables, and year to obtain
indices of abundance. We then performed
extensive checks on the robustness of our
results and tested the validity of alternative
explanations to the observed trends in abun-
dance (13). For each species, the observed
direction of the trend was the same in all
analyses, and although the magnitude of the
declines fluctuated slightly among models,
our conclusions are the same irrespective of
the model used.
We estimate that all recorded shark spe-
cies, with the exception of makos, have
declined by more than 50% in the past 8 to
15 years (Figs. 2 and 3). Although we
expect declines when populations are ini-
tially exploited, the shark populations ana-
lyzed here had been exploited to varying
degrees since the 1960s (14, 15). Because
sharks have low maximum intrinsic rates of
increase, compensatory responses to ex-
Department of Biology, Dalhousie University, Halifax,
NS, Canada B3H 4J1.
*To whom correspondence should be addressed. E-
mail: baum@mscs.dal.ca
Fig. 1. Map of the Northwest Atlantic showing the distribution of effort in the U.S. pelagic longline
fishery between 1986 and 2000, categorized by number of sets (0 to 800
⫹
), within the nine areas
assessed: 1, Caribbean; 2, Gulf of Mexico; 3, Florida East Coast; 4, South Atlantic Bight; 5, Mid
Atlantic Bight; 6, Northeast Coastal; 7, Northeast Distant; 8, Sargasso/North Central Atlantic; 9,
Tuna North/Tuna South. Areas were modified from the U.S. National Marine Fisheries Service
classification for longline fisheries. The 1000-m coastal isobath (dotted line) is given for reference.
R EPORTS
www.sciencemag.org SCIENCE VOL 299 17 JANUARY 2003 389
on December 29, 2010www.sciencemag.orgDownloaded from

ploitation are limited and recovery is ex-
pected to be slow (6 ).
The trend in abundance is most striking
for hammerhead sharks; we estimate a de-
cline of 89% since 1986 [95% confidence
interval (CI): 86 to 91%] (Figs. 2A and 3A).
This group is primarily composed of scal-
loped hammerheads (Sphyrna lewini)(16).
The trend for white sharks was an estimated
79% decline (95% CI: 59 to 89%) (Fig. 2B).
Catch rates declined in three areas that com-
prise 80% of its catch (Areas 2 to 4) (Fig.
3B). Since the early 1990s, no white sharks
have been reported in Areas 6 and 7, and very
few from Areas 5 and 8 (17). The rarity of
this species (18) resulted in less precise trend
estimates than for the other shark species.
Life-history traits have indicated that scal-
loped hammerhead and white sharks would
be among the sharks most vulnerable to over-
exploitation (19, 20).
Tiger shark catch rates declined by an esti-
mated 65% since 1986 (95% CI: 58 to 72%)
(Figs. 2C and 3C), while the coastal species
recorded from 1992 declined by an estimated
61% (95% CI: 55 to 66%) (Figs. 2D and 3D).
The latter species, members of the genus Car-
charhinus, were grouped because they are dif-
ficult to distinguish. Individual analysis, how-
ever, showed declines for each species (ranging
from 49 to 83%). Management of these species
has been a contentious issue because of uncer-
tainty in their status (21). We provide strong
quantitative evidence to support the argument
that these species have declined substantially in
the past decade.
The trends for oceanic sharks have also
shown decline. We estimate that thresher
sharks—a group composed of the common
thresher (Alopias vulpinus) and bigeye
thresher (A. superciliousus)— have declined
by 80% (95% CI: 76 to 86%) (Figs. 2E and
3E). Unlike the area examined for other oce-
anic sharks, the area examined for thresher
sharks encompasses the known distribution
of their Northwest Atlantic populations (18).
Observed declines suggest that these popula-
tions have collapsed. The interpretation of
trends in abundance for other oceanic sharks
is complex because their ranges extend across
the North Atlantic. Blue sharks declined by
an estimated 60% (95% CI: 58 to 63%) (Fig.
2F). Conflicting patterns between the areas of
highest catches (Areas 5 to 7: ⬎90% catches)
(Fig. 3F) could indicate density-dependent
habitat selection, with blue sharks moving
into preferential habitat (Area 7) as the pop-
ulation declined. Abundance of mako sharks
(mostly shortfin mako, Isurus oxyrinchus)
declined moderately (Figs. 2G and 3G). The
oceanic whitetip shark declined by an esti-
mated 70% (95% CI: 62 to 75%) (Figs. 2H
and 3H). From our data, we cannot infer
reliable trends for oceanics across the entire
North Atlantic Ocean. However, because oth-
er longline fleets exert intense fishing effort
across the North Atlantic (7), this pattern
could well be representative of the entire
region.
Our results show that overfishing is
threatening large coastal and oceanic sharks
in the Northwest Atlantic. The large and rapid
declines we document are in addition to sub-
stantial historical reductions (2, 22). Overex-
ploitation of elasmobranchs (sharks, skates,
and rays) is known to have already nearly
eliminated two skate species from much of
their ranges (23, 24). The magnitude of the
declines estimated here suggests that several
sharks may also now be at risk of large-scale
extirpation.
Marine reserves have been shown to be
effective in rebuilding depleted fish popula-
tions (25). In the open ocean this could be
different, because animals move across large
areas (26 ), as do fishing fleets (27 ). We used
simple models to analyze the implications of
large-scale marine reserves for shark conser-
vation (13). Models were based on empirical
data (distribution of fishing effort from log-
book data, catch rates per species from sci-
entific observer data) and run under two sce-
narios that represent the extremes of likely
outcomes: (i) after the closure, fishing effort
is displaced and changes such that the same
total swordfish quota is caught (“constant-
quota scenario”); or (ii) fishing effort is dis-
Fig. 2. Declines in estimated relative abundance for coastal shark species: (A) hammerhead, (B)
white, (C) tiger, and (D) coastal shark species identified from 1992 onward; and oceanic shark
species: (E) thresher, (F) blue, (G) mako, and (H) oceanic whitetip. For each species, the overall
trend (solid line) and individual year estimates (■ ⫾ 95% CI) are shown. Relative abundance is
initially set to 1, to allow comparisons among species.
Fig. 3. The estimated annual rate of change, in each area (F ⫾ 95% CI) and in all areas combined
(E ⫾ 95% CI), for coastal shark species: (A) hammerhead, (B) white, (C) tiger, and (D) coastal shark
species identified from 1992 onward; and oceanic shark species: (E) thresher, (F) blue, (G) mako,
and (H) oceanic whitetip. Areas with fewer than 40 observations are excluded.
R EPORTS
17 JANUARY 2003 VOL 299 SCIENCE www.sciencemag.org390
on December 29, 2010www.sciencemag.orgDownloaded from

placed but remains constant overall (“con-
stant-effort scenario”). Area 7 has been
closed since July 2001 to reduce bycatch of
endangered sea turtles (28). We examined the
effects of closing this area and each of the
remaining areas (Fig. 1) in turn on catches of
13 examined shark species, and on 2 turtle
and 10 finfish species of concern (29–31).
Model results show that marine reserves
can indirectly cause harm if fishing effort is
merely displaced. For example, the closure of
Area 7 meets its objective in reducing sea
turtle bycatch and also protects sharks of
lower conservation concern: blue and mako
sharks. However, this closure increases catch
of almost every other species (Fig. 4), be-
cause effort is redistributed to areas of higher
species diversity. In contrast, closure of Area
3 would afford protection to most coastal
shark species, including the hammerheads,
but catch rates of oceanic sharks and sea
turtles would increase (Fig. 4). Closure of
Area 5 would be needed to protect thresher
sharks (Fig. 4). Clearly, if marine reserves are
to be effective, their placement is of critical
importance, and conservation initiatives must
explicitly consider impacts on the whole
community of species. Emphasis on single-
species conservation, without controlling ef-
fort, simply shifts pressure from one threat-
ened species to another and may actually
jeopardize biodiversity.
We have presented strong quantitative
evidence showing large declines in many
coastal and oceanic shark species over a
short period. Our results indicate that they
should be given conservation attention
equal to that given other threatened large
marine predators. Given that in all oceans,
longline and other pelagic fisheries are in-
tense and catch many of the same shark
species (7 ), serious declining trends in
Northwest Atlantic shark abundances may
be reflective of a common global phenom-
enon. Because consumers exert important
controls on food web structure, diversity,
and ecosystem functioning (32, 33), perva-
sive overfishing of these species may initi-
ate major ecological changes. However, our
analysis shows that marine reserves are not
a panacea for overexploitation. Instead, we
suggest that carefully designed marine re-
serves in concert with reductions in fishing
effort (34) could hold promise for safe-
guarding sharks and other large pelagic
predators from further declines and ecolog-
ical extinction.
References and Notes
1. J. Alroy, Science 292, 1893 (2001).
2. J. B. C. Jackson, Proc. Natl. Acad. Sci. U.S.A. 98, 5411
(2001).
3. C. Safina, Song for the Blue Ocean (Holt, New York,
1998).
4. J. R. Spotila, R. D. Reina, A. C. Steyermark, P. T. Plotkin,
F. V. Paladino, Nature 405, 529 (2000).
5. The low fecundity and late age at maturity of elas-
mobranchs (sharks, skates, rays) render these fishes
more vulnerable to overexploitation than most te-
leost fishes, as evidenced by the history of collapsed
shark fisheries (6).
6. J. A. Musick, G. Burgess, G. Cailliet, M. Camhi, S.
Fordham, Fisheries 25, 9 (2000).
7. R. Bonfil, Overview of World Elasmobranch Fisheries
(FAO, Fisheries Technical Paper 341, Rome, 1994).
8. D. Rose, An Overview of World Trade in Sharks and
Other Cartilaginous Fishes (TRAFFIC Network, Cam-
bridge, UK, 1996).
9. J. I. Castro, C. M. Woodley, R. L. Brudek, A Preliminary
Evaluation of the Status of Shark Species (FAO, Fish-
eries Technical Paper 380, Rome, 1999).
10. We examined all available scientific observer and
logbook data for the pelagic longline fleets target-
ing swordfish and tunas in the Northwest Atlantic:
(i) U.S. observers on Japanese boats (1978 to
1988); (ii) U.S. observers on U.S. boats (1985 to
2000); (iii) Canadian observers on Japanese boats
(1979 to 1984, 1986 to 2000); (iv) Canadian ob-
servers on Canadian boats (1979 to 2000); and (v)
logbook data from U.S. boats (1986 to 2000). For
each of these data sets, we standardized catch per
unit effort (CPUE) time series using GLMs to obtain
unbiased indices of abundance. Reliable trends
could only be estimated from (v). Estimated trends
from (i) to (iv) were extremely uncertain and con-
tained much wider year-to-year fluctuations than
are realistic according to shark life histories. This is
likely the result of limited and/or nonrandom fleet
Table 1. Examined shark species, categorized as large coastal or oceanic according to the U.S. Fishery
Management Plan (FMP) for Sharks of the Atlantic Ocean (35). These species are also caught in U.S.
commercial and/or recreational shark fisheries.
Species Latin name
Year first
recorded
Total number
recorded
Large coastal species
Hammerhead spp. Sphyrna lewini, S. mokarran, S. zygaena 1986 60,402
White Carcharodon carcharias 1986 6,087
Tiger Galeocerdo cuvieri 1986 16,030
Coastal spp. Carcharhinus altimus, C. brevipinna,
C. falciformis,* C. limbatus,
C. obscurus, C. signatus
1992 80,480
Oceanic species
Thresher spp. Alopias superciliousus, A. vulpinus 1986 23,071
Blue Prionace glauca 1986 1,044,788
Mako spp. Isurus oxyrinchus, I. paucus 1986 65,795
Oceanic whitetip Carcharhinus longimanus 1992 8,526
Porbeagle Lamna nasus 1992 829
*The silky shark (C. falciformis) is biologically an oceanic species, but is classified in the FMP as a large coastal.
Fig. 4. Results from closed-area model showing
predicted changes in catch as caused by year-
round longline closure of Areas 3, 5, and 7.
Remaining areas are shown in fig. S2. Results
for the constant-quota (above and fig. S2) and
constant-effort (fig. S3) scenarios were similar.
Negative values refer to reductions in catch.
Error bars are 95% bootstrap confidence inter-
vals, accounting for the uncertainty in the ob-
server estimates of species composition. Black
bars represent sharks (SPL, scalloped hammer-
head; GHH, great hammerhead; TIG, tiger; SBG,
bignose; FAL, silky; SBK, blacktip; DUS, dusky;
SNI, night; PTH, common thresher; BTH, bigeye
thresher; BSH, blue; SMA, shortfin mako; OCS,
oceanic whitetip), dark gray bars represent sea
turtles ( TTL, loggerhead; TLB, leatherback), and
light gray bars represent finfish ( WHM, white
marlin; BUM, blue marlin; BFT, bluefin tuna;
BET, bigeye tuna; ALB, albacore tuna; DOL,
common dolphinfish; WAH, wahoo; OIL, oilfish;
SAI, Atlantic sailfish). See table S2 for scientific
names and conservation status.
R EPORTS
www.sciencemag.org SCIENCE VOL 299 17 JANUARY 2003 391
on December 29, 2010www.sciencemag.orgDownloaded from

coverage (⬍5%), or the limited temporal and spa-
tial overlap among data sets.
11. If the reporting rate has changed over time, then the
ratio of missing values to true zeros will change, and
using the assumed zeros to infer trends may lead to
biased results.
12. J. Grogger, R. Carson, J. Appl. Econ. 6, 225 (1991).
13. Methods and robustness analysis details are available
as supporting online material on Science Online.
14. H. Nakano, A Review of the Japanese Fishery and
Research on Sharks in the Atlantic Ocean (Interna-
tional Commission for the Conservation of Atlantic
Tunas, Madrid, SCRS/92/145, 1993).
15. D. Au, Species Composition in the Japanese Long-line
Fishery off Southern and Eastern United States (Inter-
national Commission for the Conservation of Atlantic
Tunas, Madrid, SCRS/84/75, 1985).
16. L. J. V. Compagno, FAO Species Catalogue, vol. 4,
parts 1 and 2, Sharks of the World: An Annotated and
Illustrated Catalogue of Shark Species Known to Date
(FAO, Rome, Fish. Synop. 125, 1984).
17. No white sharks have been caught in the 4200 sets
monitored since 1990 by the U.S. observer programs
for pelagic longline fleets in this region. Before this
time, observers had recorded 142 white sharks.
18. L. J. V. Compagno, FAO Species Catalogue for Fishery
Purposes, no. 1, vol. 2, Sharks of the World: An
Annotated and Illustrated Catalogue of Shark Species
Known to Date (FAO, Rome, 2001).
19. S. E. Smith, D. W. Au, C. Show, Mar. Freshw. Res. 49,
663 (1998).
20. NMFS, United States National Plan of Action for the
Conservation and Management of Sharks (National
Marine Fisheries Service, Silver Spring, MD, 2001).
21. Controversy over 1997 large coastal shark quota cuts
resulted in lawsuits between the fishing industry and
the U.S. National Marine Fisheries Service (NMFS). In
2002, a lawsuit protesting NMFS’s failure to protect
these species was also launched.
22. G. F. de Oviedo, Historia General y Natural de las
Indias (1535–1557) (Atlas, Madrid, reprinted 1959).
23. K. Brander, Nature 290, 48 (1981).
24. J. M. Casey, R. A. Myers, Science 281, 690 (1998).
25. B. S. Halpern, R. S. Warner, Ecol. Lett. 5, 361 (2002).
26. B. A. Block et al., Science 293, 1310 (2001).
27. L. W. Botsford, J. C. Castilla, C. H. Peterson, Science
277, 509 (1997).
28. U.S. Federal Register, vol. 66, p. 36711. This closure
remains enforced to date (U.S. Federal Register, vol.
67, p. 45393).
29. We included sea turtle and finfish species listed by
the International Union for the Conservation of Na-
ture (IUCN) and/or NMFS as severely overfished,
with further overfishing occurring, threatened, or en-
dangered (see table S2 for details). Common dolphin-
fish, wahoo, and oilfish were included because they
are caught in large numbers, but very little is known
about their stock status and sensitivity to overfishing.
All species were caught in at least two areas with a
minimum sample size of 25 individuals.
30. IUCN, 2002 IUCN Red List of Threatened Species
(IUCN, Gland, Switzerland, and Cambridge, UK, 2002)
(www.redlist.org).
31. NMFS, Stock Assessment and Fishery Evaluation for
Atlantic Highly Migratory Species 2002 (National Ma-
rine Fisheries Service, Highly Migratory Species Man-
agement Division, Silver Spring, MD, 2002).
32. T. Essington, D. Schindler, R. Olson, J. Kitchell, C.
Boggs, Ecol. Appl. 12, 724 (2002).
33. B. Worm, H. K. Lotze, H. Hillebrand, U. Sommer,
Nature 417, 848 (2002).
34. NMFS has reduced directed shark fishery quotas and
has also closed areas within the Gulf of Mexico and
along the U.S. east coast in 2000 and 2001, respec-
tively, with the aim of reducing incidental catch (of
species other than sharks) (31). No analysis has been
undertaken to test their effectiveness.
35. NMFS Final Fishery Management Plan for Atlantic
Tuna, Swordfish, and Sharks (National Marine Fisher-
ies Service, Highly Migratory Species Management
Division, Silver Spring, MD, 1999).
36. We thank NOAA-NMFS for data; J. Cramer, L. R.
Beerkircher, and J. Musick for advice; W. Blanchard,
L. Gerber, and M. Ortiz for technical assistance; H.
Keith for initial closed-area model implementation;
and H. K. Lotze and H. Whitehead for comments on
the manuscript. Many thanks to the longline fishers
who provided their logbooks to NMFS. This re-
search is part of a larger project on pelagic longlin-
ing initiated and supported by a Pew Charitable
Trusts grant to L.B. Crowder and R.A.M., with ad-
ditional support by Natural Sciences and Engineer-
ing Research Council of Canada scholarships to
J.K.B. and D.G.K.
Supporting Online Material
www.sciencemag.org/cgi/content/full/299/5605/389/
DC1
Methods
Figures S1 to S3
Tables S1 and S2
References
25 October 2002; accepted 20 November 2002
Selective Trafficking of
Non–Cell-Autonomous Proteins
Mediated by NtNCAPP1
Jung-Youn Lee,
1
Byung-Chun Yoo,
1
Maria R. Rojas,
1
Natalia Gomez-Ospina,
2
L. Andrew Staehelin,
2
William J. Lucas
1
*
In plants, cell-to-cell communication is mediated by plasmodesmata and in-
volves the trafficking of non–cell-autonomous proteins (NCAPs). A component
in this pathway, Nicotiana tabacum NON-CELL-AUTONOMOUS PATHWAY
PROTEIN1 (NtNCAPP1), was affinity purified and cloned. Protein overlay assays
and in vivo studies showed that NtNCAPP1 is located on the endoplasmic
reticulum at the cell periphery and displays specificity in its interaction with
NCAPs. Deletion of the NtNCAPP1 amino-terminal transmembrane domain
produced a dominant-negative mutant that blocked the trafficking of specific
NCAPs. Transgenic tobacco plants expressing this mutant form of NtNCAPP1
and plants in which the NtNCAPP1 gene was silenced were compromised in their
ability to regulate leaf and floral development. These results support a model
in which NCAP delivery to plasmodesmata is both selective and regulated.
In plants, the trafficking of NCAPs that are
involved in the regulation of plant development
is thought to occur through plasmodesmata (1–
6). However, little information is available con-
cerning the manner in which such NCAPs enter
this cell-to-cell translocation pathway (5, 6). To
identify potential components in this pathway,
we used the NCAP CmPP16 (7) as bait for the
affinity purification of interaction partners con-
tained within a plasmodesmal-enriched cell
wall protein (PECP) fraction (8–10) prepared
with tobacco BY-2 cells [fig. S1, A and B (11)].
A resultant highly enriched 40-kD protein was
identified (Fig. 1, fig. S1C), cloned, and named
NtNCAPP1 (GenBank accession number
AF307094; hereafter called NCAPP1) [fig. S2
(11)].
The specificity of the interaction between
NCAPP1 and CmPP16 was tested using a
protein overlay approach (11). Native
NCAPP1 (contained within the PECP prepa-
ration) interacted with only a very small sub-
set of the proteins present in the PECP frac-
tion (Fig. 2A). Furthermore, fractions en-
riched for cytoplasmic proteins exhibited
only minimal interaction with NCAPP1 (Fig.
2B). A reciprocal experiment in which the
PECP fractions were probed with the
CmPP16 bait confirmed the specificity of the
interaction between native NCAPP1 and
CmPP16 (Fig. 2C). As the CmPP16 is an
endogenous NCAP located within the phloem
sap (7 ), we next used fractionated phloem sap
(11) in an overlay with PECP and, as antici-
pated, detected a strong signal in the region
corresponding to the CmPP16 (Fig. 2D). A
range of other phloem proteins also interacted
positively with NCAPP1, consistent with ob-
servations that various phloem components
can traffic through plasmodesmata (12).
These results confirmed that the NCAPP1
enrichment achieved in our affinity chroma-
tography experiments (Fig. 1) was due to its
specific interaction with the CmPP16 bait.
The presence of a range of NCAPP1-interact-
ing proteins in the phloem sap suggests that
NCAPP1 (and other isoforms) may be central
to NCAP trafficking in general.
Subcellular localization of NCAPP1 was
examined by expression of fluorescently
tagged NCAPP1 in BY-2 cells (Fig. 3). In
contrast to the fluorescence pattern observed
with free EGFP (enhanced green fluorescent
protein) (Fig. 3A), NCAPP1-EGFP accumu-
lated at the cell periphery (Fig. 3B). Similar-
ly, fluorescence associated with CmPP16-
RFP was highest at the periphery of BY-2
cells (Fig. 3C). A role for the predicted NH
2
-
1
Section of Plant Biology, Division of Biological Sci-
ences, University of California, 1 Shields Avenue,
Davis, CA 95616, USA.
2
Molecular, Cellular, and De-
velopmental Biology, University of Colorado, Boulder,
CO 80309 – 0347, USA.
*To whom correspondence should be addressed. E-
mail: wjlucas@ucdavis.edu
R EPORTS
17 JANUARY 2003 VOL 299 SCIENCE www.sciencemag.org392
on December 29, 2010www.sciencemag.orgDownloaded from