Nanoscale organization and dynamics of SNARE proteins in the presynaptic membranes

Abstract

The specific organization of proteins and lipids in functional domains in biological membranes allows localization and segregation of specific physiological activities. Mechanisms that underlie the formation of these domains include hydrophobic and ionic interactions with membrane lipids as well as specific protein-protein interactions. Using plasma membrane-resident SNARE proteins as a model, I show that cholesterol-induced hydrophobic mismatch between the transmembrane domains and the membrane lipids not only suffices to induce clustering of proteins, but can also lead to the segregation of structurally closely homologous membrane proteins in distinct membrane domains. Domain formation is further fine-tuned by interactions with polyanionic phosphoinositides and proteins. Furthermore, Calcium ions act as a charge bridge that connects multiple syntaxin 1/PI(4,5)P2 complexes into larger domains. Segregating SNARE proteins into distinct clusters at the plasma membrane has three key functional implications for exocytosis: (i) clusters act as the local hot spots for the vesicle recruitment, (ii) the local enrichment provides sufficient number of proteins necessary for the fast, evoked synaptic release, (iii) closely homologous SNARE proteins such as syntaxin 1 and 4 are segregated in non-overlapping membrane domains which is essential for their distinct roles in regulated (syntaxin 1) and constitutive (syntaxin 4) exocytosis. Overall, the findings presented in this thesis demonstrate that the structural organization of membranes is governed by a hierarchy of interactions with hydrophobic mismatch emerging as one of the fundamental principles.