Ferlin C2A-C2B linkers are alternatively spliced, intrinsically disordered, and interact with negatively charged membranes

Abstract

Ferlins are vesicle trafficking proteins composed of folded C2 domains conjugated by linkers which are largely disordered. Although a role for the for the C2 domains as calcium sensors has been established it remains unclear whether the linkers function beyond acting as passive spacers. We examined the C2A-C2B linker of vertebrate ferlins and found both putative AP2 and SH3 binding short linear motifs (SLiMs) as well as membrane binding sequences for members of the protein family. Specifically for otoferlin we identified an arginine-rich region proximal to a AP2 binding dileucine motif which interacts with negatively charged lipid membranes. Further, the linker region dominated the liposome binding properties of a larger C2A-C2B two-C2 domain segment of otoferlin, suggesting a dominant role in mediating the membrane binding property of the N-terminus. We also found that alternative splicing of the otoferlin C2A-C2B linker adds and additional membrane binding segment and alters the affinity and kinetics of membrane binding. By contrast alternative splicing of the dysferlin linker is not predicted to alter membrane binding but rather alters the number of predicted short linear motifs (SLiMs). In addition we found the otoferlin linker-membrane interaction was sensitive to ionic strength, and simulations suggest positively charged residues including an arginine-rich region mediates binding. We conclude that the C2A-C2B linker of vertebrate ferlins encode both SLiMs which recruit endocytic proteins as well as membrane binding regions that would place the endocytic binding motif proximal to the membrane surface to facilitate endocytosis and synaptic vesicle resupply.