A unique spacer domain of synaptotagmin IV is essential for Golgi localization

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Abstract
Synaptotagmin (Syt) family members consist of six separate domains: a short amino terminus, a single transmembrane domain, a spacer domain, a C2A domain, a C2B domain and a short carboxyl (C) terminus. Despite sharing the same domain structures, several synaptotagmin isoforms show distinct subcellular localization. Syt IV is mainly localized at the Golgi, while Syt I, a possible Ca2+‐sensor for secretory vesicles, is localized at dense‐core vesicles and synaptic‐like microvesicles in PC12 cells. In this study, we sought to identify the region responsible for the Golgi localization of Syt IV by immunocytochemical and biochemical analyses as a means of defining the distinct subcellular localization of the synaptotagmin family. We found that the unique C‐terminus of the spacer domain (amino acid residues 73–144) between the transmembrane domain and the C2A domain is essential for the Golgi localization of Syt IV. In addition, the short C‐terminus is probably involved in proper folding of the protein, especially the C2B domain. Without the C‐terminus, Syt IVΔC proteins are not targeted to the Golgi and seem to colocalize with an endoplasmic reticulum (ER) marker (i.e. induce crystalloid ER‐like structures). On the basis of these results, we propose that the divergent spacer domain among synaptotagmin isoforms may contain certain signals that determine the final destination of each isoform.