Generally, synapsin tethers synaptic vesicles,
13 although it is not found in photoreceptor synapses.
13 Instead, synaptic ribbons tether vesicles with fine filaments in
photoreceptor synaptic terminals.
1 It is believed that
synaptic ribbons are involved in the transmitter release by tethering
synaptic vesicles. The present study indicates that caveolin-1 is
localized in synaptic ribbons. This finding is crucial for studying
functions of synaptic ribbons, because caveolin specifically binds to
various proteins involved in signal transduction such as G-proteins,
protein kinases,
5 6 and a calcium pump.
14 15 It is conceivable that caveolin-1 in synaptic ribbons is involved in
transmitter release by forming complexes with proteins related to
signal transduction.
Our results indicate that caveolin-1 was located in synaptic ribbons.
However, the caveolin-1 detected may be slightly different from the
protein reported previously
9 because oligomerization of
the protein was not detected by SDS-PAGE, and its molecular weight was
slightly higher than 25 kDa. It is possible that caveolin-1 found in
synaptic ribbons has a slightly different amino acid sequence (a
caveolin-1 homologue), and/or the protein in synaptic ribbons is
modified.
It is not known how synaptic ribbons are formed. One reason is
that constituents of the synaptic ribbon have not been completely
identified. Schmitz et al.
9 showed that a synaptic ribbon
fraction contains several proteins. Nguyen and Balkema
16 demonstrated that antigenic epitope against the amino acid sequence
DTYQHPPKD colocalized with α-actinin at the synaptic ribbon. Muresan
et al.
17 have indicated that KIF3A, a member of the
heteromeric family of kinesins, is a filament that tethers vesicles,
suggesting that tubulin may be involved in synaptic ribbon formation.
In this study, we found that caveolin-1 is localized in synaptic
ribbons. A 30-kDa protein found by Schmitz et al.
9 in
synaptic ribbons may be the caveolin-1 described in the present study.
Caveolin-1 was originally identified as an intrinsic membrane
protein,
4 5 suggesting that the synaptic ribbon could form
from a membrane. However, immunocytochemical data
(Fig. 2) suggest that
vesicles and vacuoles do not contain caveolin-1. Information about
incorporation of caveolin-1 into synaptic ribbons may be crucial for
revealing the mechanism of synaptic ribbon formation.
The authors thank William H. Miller for critical
reading of the manuscript.