The transmitter release-site CaV2.2 channel cluster is linked to an endocytosis coat protein complex

Synaptic vesicles (SVs) are triggered to fuse with the surface membrane at the presynaptic transmitter release site (TRSs) core by Ca2+ influx through nearby attached CaV2.2 channels [see accompanying paper: Khanna et al. (2007)Eur. J. Neurosci., 26, 547-559] and are then recovered by endocytosis. In this study we test the hypothesis that the TRS core is linked to an endocytosis-related protein complex. This was tested by immunostaining analysis of the chick ciliary ganglion calyx presynaptic terminal and biochemical analysis of synaptosome lysate, using CaV2.2 as a marker for the TRS. We noted that CaV2.2 clusters abut heavy-chain (H)-clathrin patches at the transmitter release face. Quantitative coimmunostaining analysis (ICA/ICQ method) demonstrated a strong covariance of release-face CaV2.2 staining with that for the AP180 and intersectin endocytosis adaptor proteins, and a moderate covariance with H- or light-chain (L)-clathrin and dynamin coat proteins, consistent with a multimolecular complex. This was supported by coprecipitation of these proteins with CaV2.2 from brain synaptosome lysate. Interestingly, the channel neither colocalized nor coprecipitated with the endocytosis cargo-capturing adaptor AP2, even though this protein both colocalized and coprecipitated with H-clathrin. Fractional recovery analysis of the immunoprecipitated CaV2.2 complex by exposure to high NaCl (approximately 1 m) indicated that AP180 and S-intersectin adaptors are tightly bound to CaV2.2 while L-intersectin, H- and L-clathrin and dynamin form a less tightly linked subcomplex. Our results are consistent with two distinct clathrin endocytosis complexes: an AP2-containing, remote, non-TRS complex and a specialised, AP2-lacking, TRS-associated subcomplex linked via a molecular bridge. The most probable role of this subcomplex is to facilitate SV recovery after transmitter release.