Molecular architecture of CTCFL
Amy E. Campbell1, Selena R. Martinez, JJ L. Miranda*
Department of Cellular and Molecular Pharmacology, University of California, San Francisco, San Francisco, CA 94158, USA
a r t i c l ei n f o
Received 16 April 2010
Available online 8 May 2010
a b s t r a c t
The CTCF-like protein, CTCFL, is a DNA-binding factor that regulates the transcriptional program of mam-
malian male germ cells. CTCFL consists of eleven zinc fingers flanked by polypeptides of unknown struc-
ture and function. We determined that the C-terminal fragment predominantly consists of extended and
unordered content. Computational analysis predicts that the N-terminal segment is also disordered. The
molecular architecture of CTCFL may then be similar to that of its paralog, the CCCTC-binding factor,
CTCF. We speculate that sequence divergence in the unstructured terminal segments results in differen-
tial recruitment of cofactors, perhaps defining the functional distinction between CTCF in somatic cells
and CTCFL in the male germ line.
? 2010 Elsevier Inc. All rights reserved.
The CTCF-like protein, referred to as CTCFL or BORIS, regulates
transcription in the human male germ line. The CCCTC-binding fac-
tor, CTCF, organizes transcription and chromatin in the three-
dimensional space of the nucleus [1,2]. While mammalian CTCF
is ubiquitously expressed in somatic tissue, CTCFL is a paralog ex-
pressed only during male germ cell development [3,4]. Binding of
CTCFL to differently methylated alleles suggests a role in regulating
imprinting [5–7]. Indeed, evolution of testis-specific expression
appears to correlate with the emergence of imprinting in therian
mammals . Displacement of CTCF by induced CTCFL in somatic
cells perturbs gene expression [8–10]. Given the temporal and spa-
tial separation of antagonistic functions, each paralog likely drives
distinct transcriptional programs by using dissimilar mechanisms
How do CTCF and CTCFL differentially regulate gene expres-
sion? Both proteins contain eleven central zinc fingers that are al-
most identical between the paralogs. The flanking polypeptide
segments, which comprise about half of each protein, show very
little sequence homology between CTCF and CTCFL [3,4]. One
might therefore suspect that the divergent functions of each pro-
tein arise from differences in the terminal extensions. Our efforts
to elucidate the structures of these polypeptides revealed that both
fragments of CTCF are unstructured . We have now learned
that the C-terminal fragment of CTCFL is also unstructured.
Computational analysis predicts that the N-terminal segment
may be disordered as well. We discuss the functional implications
of a similar structural architecture on the functional differences be-
tween CTCF and CTCFL.
2. Materials and methods
2.1. Protein expression and purification
The terminal segments of CTCFL were defined as the polypep-
tides flanking the eleven zinc fingers predicted by a minimum con-
sensus motif . The gene fragment encoding the C-terminal
segment was obtained by PCR amplification from human cDNA
clone SC311151 (Origene). This amplicon contained an NdeI site,
the sequence encoding amino acids 571–663, the sequence
WSHPQFEK, a TAA stop codon, and a BamHI site, all of which
was inserted between the NdeI and BamHI sites of pET-15b
(EMD Biosciences) to generate pAEC38. Additional sequences
flanking the gene fragment were included by incorporation in
PCR primers. The construct encodes both an N-terminal His-tag
and a C-terminal Strep-tag. Sequencing of the open reading frame
(UCSF Genomics Core Facility) confirmed proper cloning of the
reference sequence . The C-terminal fragment of CTCFL was ex-
pressed in bacteria and purified with affinity and size-exclusion
chromatography in the same manner as the C-terminal fragment
of CTCF .
that encodes theStrep-tag
Hydrodynamic radii were determined using size-exclusion
chromatography . Circular dichroism was measured with a
0006-291X/$ - see front matter ? 2010 Elsevier Inc. All rights reserved.
* Corresponding author.
E-mail address: firstname.lastname@example.org (JJ L. Miranda).
1Present addresses: Division of Hematology, The Children’s Hospital of Philadel-
phia, Philadelphia, PA 19104, USA; University of Pennsylvania School of Medicine,
Philadelphia, PA 19104, USA.
Biochemical and Biophysical Research Communications 396 (2010) 648–650
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journal homepage: www.elsevier.com/locate/ybbrc
J-715 spectropolarimeter (Jasco) equipped with a PTC-348WI
Peltier temperature control system (Jasco). Purified protein was
exchanged into 25 mM sodium phosphate, 250 mM NaF, 1 mM
b-mercaptoethanol, pH 7.5 by repeated concentration and dilution
in a centrifugal filter unit. Spectra were acquired with 10 lM pro-
tein in a 1 mm cuvette (Hellma) at 20 ?C. The spectrum of buffer
alone was also acquired and subtracted from the spectrum of
the protein solution. Fractional secondary structure content was
estimated with the CONTIN/LL and CDSSTR algorithms of CDPro
 using data from 200 to 240 nm compared to reference set
2.3. Computational analysis
Disordered regions of CTCFL were predicted by examining the
protein primary sequence. Analysis was performed with the
PONDR VL-XT algorithm .
3. Results and discussion
3.1. Biophysical properties of the CTCFL C-terminal fragment
The hydrodynamic radius of the CTCFL C-terminal fragment is
larger than that expected for a globular protein. The recombinant
C-terminal polypeptide can be purified from bacteria as a single
migrating band on an SDS–PAGE gel (Fig. 1A) and as a single elut-
ing peak during size-exclusion chromatography (Fig. 1B). Because
of previous experience with unstructured fragments of CTCF, we
tested this CTCFL polypeptide for behavior that suggests the lack
of a folded domain. We therefore measured hydrodynamic proper-
ties with size-exclusion chromatography (Fig. 1B). Unstructured
proteins have larger Stokes radii than compact folds because
extended conformations generate more drag in solution. The CTCFL
C-terminal fragment yields a Stokes radius of 26.2 ± 0.3 Å (N = 6). A
13 kDa protein is expected to migrate with a radius of 19 Å if glob-
ular and 32–34 Å if unfolded . We excluded oligomerization as
a cause of increased drag; the molar mass of the purified polypep-
tide as observed by multi-angle light scattering coupled with size-
exclusion chromatography is close to that expected for a monomer
(data not shown). A larger than expected hydrodynamic radius
suggests that the C-terminal fragment could be unstructured, but
determination of the amount of secondary structure is needed to
verify this deduction.
The C-terminal fragment of CTCFL is predominantly unordered.
We estimated secondary structure content by measuring circular
dichroism. The far UV spectrum (Fig. 2) contains a strong minimum
at ?199 nm and is very similar to that of random coils, which con-
tains a minimum at ?197 nm . a helices or b strands yield dis-
tinct minima above 200 nm, none of which are seen here. We then
calculated the fractional secondary structure content by compari-
son to proteins of known structures . The CONTIN/LL algorithm
estimated 4 ± 0% helix, 12 ± 2% strand, 6 ± 1% turns, and 78 ± 2%
unordered content (N = 3). Analysis with the CDSSTR algorithm cal-
culated similar results, estimating 3 ± 0% helix, 15 ± 2% strand,
10 ± 1% turns, and 71 ± 4% unordered content (N = 3). We note that
while determining the exact unordered content of polypeptides
suffers from multiple computational challenges, the rough esti-
mates are informatively accurate [17,18]. Our observations can
be interpreted as concluding that the C-terminal fragment consists
mostly of unstructured content.
3.2. Sequence analysis of the CTCFL N-terminal segment
Computational analysis predicts that the N-terminal segment of
CTCFL may be disordered. We could not purify native preparations
of the N-terminal fragment sufficiently well behaved for biochem-
ical experiments. Although we would have preferred to obtain
experimental data, we consequently turned to computational
methods to identify unstructured portions of CTCFL. Sequence
analysis trained on known ordered and disordered polypeptides
 predicts that a significant portion of the N-terminal segment
may be unstructured (Fig. 3). Correct identification of disorder in
the rest of CTCFL gives us confidence in this analysis. The C-termi-
nal segment is predicted to be unstructured as experimentally ob-
served. The probability of disorder in each zinc finger is
appropriately low, but much higher in the linkers between some
domains. Moreover, this same algorithm also correctly identifies
the ordered and disordered regions of CTCF (data not shown). If
one accepts the computational prediction, then both terminal seg-
ments of CTCFL may be unstructured.
Fig. 1. Purification of the CTCFL C-terminal fragment. (A) Purified protein resolved
on a 10–20% SDS–PAGE gel. Positions of molecular mass standards are indicated
on the left. (B) Size-exclusion chromatography. Red and blue lines represent UV
absorbance at 260 and 280 nm, respectively. V0 denotes the excluded void
volume. Green triangles mark elution times of standards with known Stokes
Fig. 2. Circular dichroism of the CTCFL C-terminal fragment. Far UV spectrum.
A.E. Campbell et al./Biochemical and Biophysical Research Communications 396 (2010) 648–650
3.3. Functional implications of CTCFL molecular architecture
The unstructured terminal segments of CTCF and CTCFL could
recruit different binding partners to chromosomes. With CTCF,
we found that no domains, defined biochemically as autono-
mously folding units of stable secondary structure, exist in the ter-
minal extensions . Such an architecture limits possible
functions of the terminal segments to that observed with other
unstructured polypeptides, which is predominantly molecular rec-
ognition . With CTCFL, the C-terminal fragment consists
mostly of unordered content, and the N-terminal segment is pre-
dicted to be disordered. CTCF and CTCFL thus appear to have a
similar molecular architecture, eleven zinc fingers flanked by
unstructured extensions. The zinc fingers are strongly conserved
between paralogs, but the terminal segments are significantly di-
verged [3,4]. Lack of sequence homology suggests that if the
unstructured extensions of CTCF and CTCFL function in molecular
recognition as we surmise, then perhaps each paralog binds differ-
ent proteins to mediate distinct effects on chromatin and tran-
scription. Although both factors may bind the same DNA
sequence, the two could recruit dissimilar activities to each site.
We hypothesize that differential assembly of regulatory proteins
by the unstructured terminal segments of CTCF and CTCFL could
explain how each paralog directs specific genetic programs in so-
matic cells and the male germ line.
We thank Meghan M. Holdorf for insightful discussions and
Brian K. Shoichet for use of the circular dichroism spectropolarim-
eter. Our research is supported by an institutional grant to JJ L.
Miranda from the UCSF Fellows Program, which is funded in part
by the UCSF Program for Breakthrough Biomedical Research and
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Fig. 3. Computational prediction of disordered regions in CTCFL. Analysis with the
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A.E. Campbell et al./Biochemical and Biophysical Research Communications 396 (2010) 648–650