Differential structure and functional roles of domain E in synapsin 1 and 2

Synapsins are a family of synaptic vesicle (SV)-associated phosphoproteins involved in several physiological processes in neurons, including synapse formation and function. In the presynaptic compartment, synapsins have a key role in the maintenance of the reserve pool (RP) of SVs through condensation mediated by liquid-liquid phase separation (LLPS). The human synapsin family is formed by three paralog genes (SYN1-3), each encoding a protein (Syn1-3) with multiple isoforms. The ‘a’ isoform of all three synapsin proteins contains a C-terminal E domain which is involved in the regulation of SV pools and neurotransmitter release. This domain has been generally thought to be structurally and functionally similar across the three paralog proteins. Here, we tested whether synapsin domains E may have instead isoform-specific structural and functional features, focusing specifically on those of human Syn1a and Syn2a. We found that the E domains of Syn1a and Syn2a have distinct primary sequence features and structure, with a differential ability to form α-helical coiled-coils. These paralog-specific features regulate the ability to undergo LLPS of the E domains in the cellular context, when expressed as such or as part of their parent protein. Finally, we identify a putative SUMOylation site in the domain E of Syn2a that may be important in the physiological regulation of LLPS. These findings shed new light on the structure and function of domains E in Syn1 and Syn2, uncovering their role in mediating LLPS, and contribute to our mechanistic understanding of the paralog-specific functional roles of synapsins in neurons.