Internalization signals in synaptotagmin VII utilizing two independent pathways are masked by intramolecular...

The synaptotagmin family of membrane proteins has been implicated in both exocytosis and endocytosis. Synaptotagmin I, a protein containing two tandem C2 domains (the C2A and the C2B) in its cytoplasmic tail, is involved in regulated exocytosis of synaptic vesicles as well as compensatory endocytosis. A related family member, synaptotagmin VII, is involved in multiple forms of regulated exocytosis of lysosomes and secretory granules. In this study we show that the cytoplasmic C2 domains in synaptotagmin VII contain unique internalization signals and regulators of these signals. The C-terminal portion of the C2B is internalized in much the same way as the corresponding region of synaptotagmin I. This signal is tryptophan-based and dynamin and eps15 dependent. In contrast, the C2A contains an unusual internalization signal that is not seen in the C2A of synaptotagmin I. This signal is not based on the homologous tryptophan in its C-terminus. Moreover, internalization of the C2A domain is both dynamin and eps15 independent. Finally, the C2B domain of synaptotagmin VII contains an inhibitory motif that prevents internalization. Endocytic trafficking of synaptotagmin VII is thus governed by these two latent internalization signals, which are concealed by intramolecular inhibition. We propose that endocytosis of synaptotagmin VII is regulated in this way to allow it to couple the processes of regulated exocytosis and compensatory endocytosis.