Investigating Synaptophysin Targeting to the Photoreceptor Synapse

Abstract

Synaptophysin (SYP) is the most abundant protein found in synaptic vesicles. It functions to regulate the endocytosis phase of the synaptic vesicle cycle. However, it is not known how newly synthesized SYP is targeted from the neuronal soma to the synapse so that it can carry out this function. The photoreceptor is an excellent neuron in which to tackle this problem, due in part to its large size and clear compartmentalization. The objective of this study is to test the hypothesis that SYP contains a sequence specific targeting signal that directs the targeting of this protein to the synaptic terminal in photoreceptors. Our approach was to generate constructs consisting of a membrane‐associated YFP as a reporter fused to various regions of SYP, and express those constructs in the rod photoreceptors of transgenic Xenopus laevis. The subcellular localization of these proteins in tadpole photoreceptors was assessed by confocal microscopy. The YFP reporter was found throughout the cell but strongly accumulated in the outer segment, as we have previously described for membrane proteins lacking targeting information in this cell type. We found that only the cytoplasmic C‐terminus of SYP was capable of restricting the localization of the reporter to the inner segment plasma membrane and synaptic terminal. Future studies are aimed at determining which of SYP's binding partners regulates its trafficking in photoreceptors.