Role of the conserved WHXL motif in the C terminus of synaptotagmin in synaptic vesicle docking

Synaptotagmin (Syt) I, an abundant synaptic vesicle protein, consists of on e transmembrane region, two C2 domains, and a short C terminus. This protei n is essential for both synaptic Vesicle exocytosis and endocytosis via its C2 domains. Although the short C terminus is highly conserved among the Sy t family and across species, little is known about the exact role of the co nserved C terminus of Syt I. In this paper, we report a function of the Syt I C terminus in synaptic vesicle docking at the active zones. Presynaptic injection of a peptide corresponding to the C-terminal 21 amino acids of Sy t I (named Syt-C) into the squid giant synapse blocked synaptic transmissio n without affecting the presynaptic action potential or the presynaptic Ca2 + currents. The same procedure repeated with a mutant C-terminal peptide (S yt-CM) had no effect on synaptic transmission. Repetitive presynaptic stimu lation with Syt-C produced a rapid decrease in the amplitude of the postsyn aptic potentials as the synaptic block progressed, indicating that the pept ide interferes with the docking step rather than the fusion step of synapti c vesicles. Electron microscopy of the synapses injected with the Syt-C pep tide showed a marked decrease in the number of docked synaptic vesicles at the active zones, as compared with controls. These results indicate that Sy t I is a multifunctional protein that is involved in at least three steps o f synaptic vesicle cycle: docking, fusion, and reuptake of synaptic: vesicl es.