Molecular Motors: Strategies to Get
Roop Mallik and Steven P. Gross
The majority of active transport in the cell is driven by
three classes of molecular motors: the kinesin and
dynein families that move toward the plus-end and
minus-end of microtubules, respectively, and the
unconventional myosin motors that move along actin
filaments. Each class of motor has different proper-
ties, but in the cell they often function together. In
this review we summarize what is known about their
single-molecule properties and the possibilities for
regulation of such properties. In view of new results
on cytoplasmic dynein, we attempt to rationalize how
these different classes of motors might work
together as part of the intracellular transport machin-
ery. We propose that kinesin and myosin are robust
and highly efficient transporters, but with somewhat
limited room for regulation of function. Because
cytoplasmic dynein is less efficient and robust, to
achieve function comparable to the other motors it
requires a number of accessory proteins as well as
multiple dyneins functioning together. This necessity
for additional factors, as well as dynein’s inherent
complexity, in principle allows for greatly increased
control of function by taking the factors away either
singly or in combination. Thus, dynein’s contribution
relative to the other motors can be dynamically
tuned, allowing the motors to function together dif-
ferently in a variety of situations.
Cells are organized with different compartments — the
nucleus, the Golgi complex, the endoplasmic
reticulum, and so on — that act as factories. Each
factory generates a unique set of products, which are
then distributed to ‘consumers’, which could be either
end-users or other factories. The distribution system
is complex, and uses three sets of molecular
transporters: the myosin, kinesin and dynein motors.
Intracellular transport occurs along two sets of paths,
both of which are similar to rail systems: the more or
less randomly oriented actin filaments, used by
myosin; and the (typically) radially organized micro-
tubules used by both kinesin and dynein. Transport
occurs along each of these when the appropriate
motor binds to a cargo through its ‘tail’ and simulta-
neously binds to the rail through one of its ‘heads’
(Figure 1). The motor then moves along the rail by
using repeated cycles of coordinated binding and
unbinding of its two heads, powered by energy
derived from hydrolysis of ATP (reviewed in [1–4]).
Microtubules are polar, and are typically organized
with ‘minus ends’ clustered at a microtubule-
organizing center situated close to the nucleus. The
microtubule ‘plus ends’ spread outwards from the
organizing center, and this leads to a radial organiza-
tion (see Figure 1A) of the microtubule network in
some interphase cells, such as fibroblast cells ,
pigment cells [6,7] and certain mammalian cells .
Microtubule organization is cell-type specific and in
some cases, such as neurons  and epithelial cells
, differs significantly from the radial organization
shown in Figure 1A; the microtubule organization in
neurons is shown schematically in Figure 1B.
Motor proteins are able to recognize the
microtubule polarity, and so the organization of the
rails combined with the specific motor employed
determines the direction of transport. Most kinesin-
family motors that have been studied move toward the
plus-end of the microtubules [1,2,11], and thus
kinesin-mediated transport is usually used to bring
cargos toward the cell periphery. In contrast, dynein
moves in the other direction — toward the microtubule
minus-end [1,2,12] — and is typically used to move
cargos toward the cell center (and nucleus).
Actin filaments are more randomly oriented, and
can be used by unconventional myosin motors, such
as myosin-V, to ferry cargos . Actin filaments are
significantly shorter than microtubules [6,14] and have
been suggested to bridge the gap between micro-
tubules, for example in cultured rat axons [15,16]. In
this way, local transport can occur on actin filaments
in regions where there are few microtubules, as at the
axon terminal . As with microtubules, the organi-
zation and density of actin filaments is cell-type
specific. In some cases, actin filaments have an
ordered structure close to the cell surface [17,18] with
barbed (plus) ends pointed outwards, which could
allow myosin-V — which moves toward the actin
filament plus end — to transport cargos to the very
edge of the cell.
In one example of how transport might work, it has
been suggested that kinesin-mediated transport
brings vesicles to neuronal termini, at which point the
kinesins are degraded and the vesicles are
subsequently transported along actin filaments by
myosin-V in the actin-rich neuron terminus [13,19]. In
contrast, during endocytosis, myosin-VI — which
moves in the opposite direction from myosin-V,
toward actin filament minus ends  — can be used
to bring recently internalized cargos into the cell .
At least in some cases, however, further inside cells
the actin filament network is approximately randomly
oriented and has sufficient density to make it a good
local transport system [17,22]. This random distribu-
tion of actin filaments can be used to spread out
cargos , enabling the cell to achieve a more
uniform distribution of cargos than would be possible
by moving on microtubules alone [7,23].
Current Biology, Vol. 14, R971–R982, November 23, 2004, ©2004 Elsevier Ltd. All rights reserved. DOI 10.1016/j.cub.2004.10.046
Department of Developmental and Cell Biology, University of
California Irvine, California 92697, USA.
In some systems, the same cargo can move on both
microtubule and actin filaments, switching between
motors in the course of motion. Cargos moving this
way include pigment granules [6,7], axonal vesicles
[24,25], mitochondria  and endosomes [27,28]; for
reviews, see [16,24]. A functional collaboration 
can then exist between microtubule and actin filament
networks, and there have been suggestions that
motors associated with each network coordinate to
achieve the requisite subcellular distribution of cargo
[13,16,24]. At a global level, therefore, the intracellular
transport machinery appears to regulate the relative
activity of different classes of motors.
Surprisingly, motors also often appear to work
together locally — intracellular transport often employs
multiple motors of different classes on the same
organelle. For example, multiple dyneins and kinesins
attach to, and move, single lipid droplets along micro-
tubules in bidirectional (back and forth) fashion inside
the syncytial Drosophila embryo [30,31]. Such a strat-
egy seems quite widespread [23,32–43] though it is not
clear why an energy-inefficient mode of transport with
oppositely inclined motors is necessary. So to under-
stand intracellular transport, we have to understand
both how the activity of individual motors can be con-
trolled, and also how a certain class of motors is regu-
lated with respect to another class.
Two complementary approaches to such research
can be visualized. In a ‘top-down’ approach, within a
complex and intact transport system, one could
investigate how motor activity is controlled to achieve
net regulated motion. Here, an in vivo system is
typically under investigation, such as lipid droplets in
Drosophila  or pigment granules in melanophores
. Attempts are made to both understand particular
molecular interactions, and also model the system
dynamics in all its complexity . Recent reviews
[45,46] summarize what is known from such
approaches. In contrast, in a ‘bottom-up’ approach,
one can start from single-molecule properties of the
motors themselves and attempt to understand what
specific adaptations of each motor make it amenable
to regulation by the cellular transport machinery. This
review takes the latter approach. By summarizing how
individual motor function can be regulated, we
develop a hypothesis about how these properties
allow motors to work well together.
A review now appears appropriate in view of recent
results [47–53] on dynein. As our understanding of this
most complex of motors evolves, we can consider
other motors in a new light. We begin with a brief
summary of the kinesin and myosin motors, though
the interested reader should consult several excellent
reviews [2–4,54–56] for further details. We then
discuss dynein, emphasizing how it differs in funda-
mental ways from kinesin and myosin. The implica-
tions of these differences are discussed in the spirit of
the aforementioned bottom-up approach. We con-
clude with a discussion of how these different motors
might fit into the bigger picture of cellular transport.
As properties of the processive organelle transporters
kinesin-1, myosin-V and now cytoplasmic dynein are
better understood, we present this review in the spirit
of understanding how these three motors might work
Figure 1. Organization of microtubules in a eukaryotic cell.
(A) An interphase fibroblast-type cell showing the roughly radial arrangement of microtubules (dark lines). Microtubules nucleate at the
organizing center (green), with their fast-growing plus ends extending toward the cell periphery. A few different forms of cargo and asso-
ciated molecular motors are also shown. (B) A neuronal cell, showing the organization and polarity of microtubules within the axon and
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