Identification of syntaxin-1A sites of phosphorylation
by casein kinase I and casein kinase II
Thierry Dubois
1
, Preeti Kerai
2,
*, Michele Learmonth
1
, Andy Cronshaw
1
and Alastair Aitken
1
1
The University of Edinburgh, Division of Biomedical and Clinical Laboratory Sciences, UK;
2
Division of Protein Structure,
National Institute for Medical Research, London, UK
Casein kinases I ( CKI) are s erine/threonine protein k inases
widely expressed in a range of eukaryotes including yeast,
mammals and plants. They have been shown to play a role in
diverse physiological events including membrane trafficking.
CKIa is associated with synaptic vesicles and phosphory-
lates s ome synaptic vesicle associated proteins including
SV2. In this report, we show that syntaxin-1A is phospho-
rylated in vitro by CKI on Thr21. Casein kinase II (CKII)
has been shown previously to phosphorylate syntaxin-1A
in vitro and w e h ave i dentified Ser14 a s the CKI I phospho-
rylation site, w hich is known to be phosphorylated in vivo.
As syntaxin-1A p lays a k ey role in the regulation of n euro-
transmitter release by forming part o f the SNARE (soluble
N-ethylmaleimide-sensitive factor attachment protein

In this report, we have identified s yntaxin-1A as a
novel substrate for CKI, which further supports a role
for CKI in membrane trafficking. Indeed, the involve-
ment o f syntaxin-1A in n eurotransmitter release is well
documented (reviewed in [16,17]). Regulated n eurotrans-
mitter secretion is the key s tep in synaptic transmission
and is the basis of intercellular communication in the
nervous system. Synaptic vesicle exocytosis is regulated
by Ca
2+
and by a large number of proteins (reviewed in
[17,18]). Syntaxin-1A is associated with the presynaptic
membrane and associates with the plasma membrane
protein SNAP-25 and the synaptic vesicle protein syna-
ptobrevin to form a ÔSNARE complexÕ.Assemblyofthis
complex is necessary and may be sufficient to trigger
membrane fusion (reviewed in [17]).
Syntaxin-1A has been previously shown to b e phospho-
rylated in vitro by casein kinase II (CKII) [19–21]. Although
the site of phosphorylation was not identified, Ser14 was
speculated to be the phosphorylation site as it is present
within a CKII consensus motif. Recently, it has been shown
using phospho-specific antibodies that Ser14 i s phosphory-
lated in vivo [22]. H owever, the kinase responsible for this
was not identified. Here, we h ave identified the in v itro
phosphorylation sites within syntaxin-1A that are phospho-
rylated by both CKI and CKII. CKI and CKII phospho-
rylate the N-terminal domain o f syntaxin-1A o n Thr21 and
on Ser14, respectively.
MATERIALS AND METHODS

[15,23]. Escherichia coli DH5 a cells containing the over-
expressing plasmid w ere grown overnigh t at 37 °Cin
500 mL Luria-Bertani broth containing 100 lgÆmL
)1
ampi-
cillin. Cultures w ere diluted 1 0-fold with fresh medium and
allowed to grow until a D
600
 0.6 was reached. Expression
of GST–syntaxin-1A ( 1–265, 1–190 and 191–265) was
induced with 0.5 m
M
isopropyl thio-b-
D
-galactoside (IPTG,
Biogene Ltd) for 4 h at 37 °C. Bacterial cells were harvested
and resuspended in 150 mL sonication buffer (NaCl/P
i
,
1m
M
EDTA, 5 m
M
dithiothreitol, 1 m
M
benzamidine,
2 lgÆmL
)1
aprotinin and 1 m
M

PAGE. Syntaxin was stored in 20 m
M
Tris/HCl pH 7.5,
150 m
M
NaCl, 10% glycerol at )20 °C and protein
concentration was estimated using the Bio-Rad protein
assay. Cleavage of the GST fusion construct w ith thrombin
resulted in an addit ional five amino acids at the N-terminus
(GSPEF).
Munc18-1 was purified essentially as described for
syntaxin, e xcept for the following. Expression of GST–
Munc18-1 was i nduced with 1 m
M
IPTG for 4 h at
30 °C. Munc18-1 was further purified using gel filtration
on a S-200 column. The column was washed and
equilibrated with 20 m
M
Tris/HCl pH 8, 200 m
M
NaCl.
Munc18-1 was analysed o n 12.5% SDS/PAGE. F rac-
tions containing munc18-1 were s tored in 20 m
M
Tris/
HClpH8,200m
M
NaCl, 10% glycerol at )70 °C.
Cleavage of the GST fusion construct with thrombin

that play prominent roles in many biological processes
including synaptic vesicle exocytosis [17,25]. For example,
the phosphorylation of synaptic vesicle-associated protein
SV2 by CKI modulates its ability to interact with synap-
totagmin. Phosphorylation of syntaxin-1A by CKII increa-
ses its ability to i nteract with s ynaptotagmin [21]. I n addition,
the phosphorylation of m unc18 by PKC [26] and cdk5 [27]
leads to a significantly reduced affinity for s yntaxin-1A.
A role f or CKI i n exocytosis has been proposed as it is
associated with synaptic vesicles and phosphorylates SV2
[12,13]. In a ddition, we have found that CKI interacts with
centaurin-a
1
[15], a protein that associates with presynaptic
vesicular structures [14]. Therefore, we examined whether
CKIa was capable of phosphorylating syntaxin-1A.
Figure 1 shows that the cytoplasmic tail of syntaxin-1A
(residues 1–265) is ph osphorylated by recombinant CKIa.
Munc18, a protein which interacts with syntaxin-1A and
regulates SNARE complex f ormation, is not a s ubstrate for
CKIa (Fig. 1). 14-3-3 f is phosphorylated by CKI [23] and
was used as a positive c ontrol. CKIa was unable to
phosphorylate GST or centaurin-a
1
, which were used as
negative controls [15]. T herefore, these results indicate that
CKIa specifically phosphorylates syntaxin-1A.
In agreement with our data in Fig. 1, CKII has been
reported to phosphorylate syntaxin-1A in vitro [19–21].
GST and cas ein were used as negative and positive

CKII nor PKA.
We then identified the sites on syntaxin-1A that are
phosphorylated by CKI a nd CKII. Initially two truncated
mutants of syntaxin-1A (1–190 and 191–265) were subjected
to in vitro kinase assays. Syntaxin-1A (1–190), but not
syntaxin-1A (191–265), is a substrate for both kinases
indicating th at the phosphorylated residue(s) a re located
within t he N-terminal moiety of syntaxin-1A (Fig. 2). Our
data support previous reports indicating that CKII phos-
phorylates the N-terminal 75 amino acids of syntaxin-1A
while a fragment containing residues 76–265 is not phos-
phorylated [19]. Risinger & Bennett obtained preliminary
evidence that the CKII phosphorylation s ite is within
residues 8–75 [21]. This region contains three potential
phosphorylation sites for CKII (Ser10, Ser14 and Thr71).
These authors reported that phosphorylation occurred on
both serine and threonine residues, thus suggesting that
Thr71 could be one of the residues phosphorylated by CKII
[21]. However our mass spectrometry results clearly show
that the p eptide including this threonine is not phosphory-
lated by CKII. The peptide consisting of residues 71–84 was
identified by mass spectrometry exclusively as an unpho-
sphorylated peptide of mass 1692.9 Da (M + H)
+
.No
32
P
radioactivity was associated with this peptide from the
HPLC (data not shown).
To identify the site(s) of syntaxin-1A that a re phospho-

coupled peptides a fter phosphorylation by C KIa and
purification by HPLC. The radioactivity was released in
Fig. 1. Phosphorylation of centaurin-a
1
, 14-3-3, syntaxin-1A, and
munc18 by CKI, CKII and PKA. Casein kinase I a (CKIa,upper
panel), casein kinase II (CKII, middle panel) or protein kinase A
(PKA, bottom panel) were tested for their ability to phosphorylate
in vitro 20 pmoles of centaurin-a
1
, munc18, s yntaxin 1 A or 14-3-3
proteins. Histone H1, 14-3-3 f and c asein were used as p ositive controls
for PKA, CKI a nd CKII, respectively. The positions of p hosphory-
lated m unc18 and syntaxin 1 A are in dicated. T he weak signals
observed with syntaxin-1A by PKA and m unc18 b y CKI represent
background and nonspecific phosphorylation. The positions of the
molecular mass markers (k Da) are indicated.
Fig. 2. Phosphorylation of syntaxin-1A (1–265, 1–190, 191–265) by
CKI and CKII. In order to map the CKI and C KI I phosphorylation
site(s) w ithin syntaxin 1 A, 2 lg of the c ytoplasmic domain (residues
1–265) or the truncated versions (1–190 and 191–265) of syntaxin 1 A
were tested for their ability to be phosphorylated by CKIa (left panel)
or CKII (right panel). The positions of the molecular mass markers
(kDa) are indicated.
Ó FEBS 2002 Phosphorylation of syntaxin-1A by CKI and CKII (Eur. J. Biochem. 269) 911
cycle 9 corresponding to residue Thr21 and this residue
represents the only amino acid phosphorylated within the
peptide. A total of three separate sequencing runs were
carried out on phosphopeptides from the two constructs.
Similarly, we identified the residue(s) of syntaxin-1A

Edman degradation on covalently coupled peptides after
phosphorylation of syntaxin-1A (1–265) by CKII and
purification by HPLC in dicates that Ser14 (corresponding
to cycle 5) w as the site of phosphorylation (Fig. 5).
Radioactivity associated with cycle 1 (Thr10) was due to
incomplete washing of the radioactivity associated nonspe-
cifically to the d isc (see Fig. 6). This result indicates th at there
is only one residue (Ser14) phosphorylated within the
peptide. Identical results were obtained with syntaxin-1A,
1–190 (data not shown). Two separate sequencing runs were
carried out on phosphopeptides from each of the constructs.
Because the site of phosphorylation by CKII was close to
the N-terminus of syntaxin-1A, automated Edman degra-
dation of the syntaxin ( 1–265) which had not been digested
with trypsin w as carried out. Surprisingly this gave two
sequences in a ratio of 2 : 1 of the shorter form (Fig. 6). This
Fig. 4. CKIa phosphorylates in vitro syntaxin 1 A on Thr21. Release of
32
P at each cycle of automatic Edman degradation after phosphory-
lation of syntaxin-1A by C KIa and purification by HPLC. Peptides
(fraction 16 from Fig. 3) were covalently coupled to arylamine mem-
brane and se quen ced i n an A BI 477 s eq uencer [24]. S eq uence d ata
from one of three runs is shown. Result indicates that the radioactivity
incorporated into the peptide is released in cycle 9 corresponding to
Thr21. This residue was the only one to be phosphorylated within the
peptide.
Fig. 3. HPLC trac e (absorbance a t 210 nm) with positions of
32
P-labelled tryptic peptides of syntaxin-1A ( 1–190) phosphorylated by
CKIa. A C-terminal truncated con struct of mutant of syntaxin-1A

the consensus cleavage site to be cleaved by thrombin which
has a preference for R-X peptide bonds.
Thrombin cleavage site, LVPR/GSPEFM
1
KDR…;
additional cleavage observed, QELR/TAK DSDDDD
… (where T i s T10).
The direct sequen cing b y E dman degradation of the
protein showed a burst of radioactivity at cycle 5 which
corresponds to Ser14 (from the additional thrombin
cleavage site). No counts were observed a t cycle 15 (i.e.
Thr10) verifying that the
32
P released at cycle 1 ( Fig. 5) was
due to incomplete washing of nonspecific radioactivity
associated with the disc. A ÔburstÕ of counts was also
observed at cycle 19 corresponding to Ser14 from the
sequencing of the protein without the additional cleavage
site (Fig. 6 ).
The sites of phosphorylation by C KIa (Thr21) and CKII
(Ser14) on syntaxin-1A are summarized in Fig. 7 with the
masses of the peptides identified.
To summarize, we have shown that CKIa and CKII
phosphorylate syntaxin-1A on Thr21 and on Ser14,
respectively. Both sites are located at the N-terminal domain
of syntaxin-1A. T hese sites are remote from the H3 domain
required for protein–protein interactions, and known to be
crucial in the formation of the four-helix bundle s tructure
that it is part of the SNARE complex involved in the fusion
process of synaptic vesicles with the plasma membrane.

the PTH-amino acids released at e ach cycle
(Ôcycle numberÕ) are shown. The top
sequence (ÔExpected TCÕ, for expected
thrombin cleavage) c orresponds to the
ÔintactÕ prot ein after thrombin digestion. The
bottom sequence corresponds to the shorter
protein obtained from t he additional
thrombin c leavage site (ÔAdditional TCÕ).
Ó FEBS 2002 Phosphorylation of syntaxin-1A by CKI and CKII (Eur. J. Biochem. 269) 913
responsible for phosphorylating this site in vivo. Risinger &
Bennett proposed that both serine and threonine residues
were phosphorylated by CKII within residues 8–75 of
syntaxin-1A [21], but we have no evidence for any other
additional phosphorylation sites apart from Ser14.
Previous reports have suggested a role for CKI in
synaptic vesicle exocytosis as i t i s known to associate with
synaptic vesicles [12], and to phosphorylate the synaptic
associated protein SV2 [12,13]. Our data further support a
role for CKI in synaptic vesicle cycling. Phosphorylation o f
syntaxin-1A by CKI and CKII m ay play a prominent role
in the regulation of neurotransmitter release.
ACKNOWLEDGEMENTS
The authors would like to thank Pietro de Camilli for the munc18-
pGEX2T plasm id and Teruo Abe for the various syntaxin constructs
used in this study. This w ork was supported by a Medical Research
Council P rogramme Grant (to A. A.) a nd by the Medical Research
Council (to P . K.).
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