(204n) Capturing the Membrane-Triggered Conformational Transition for Pore Forming Cytolysin (ClyA) Using Structure Based Models

Pore forming proteins lyse target cells by rapid and unregulated pore formation on the plasma membrane and are widely implicated in a variety of virulent bacterial diseases. Cytolysin a (ClyA) is an α-pore forming proteins which undergoes a large conformation change upon binding to the target membrane. The rate determining step of this process is the conversion of the water soluble Clya monomer into a membrane inserted protomer. We elucidate the mechanism of this conformational transition using molecular dynamics simulations of coarse-grained structure based (GO) models of Clya and a membrane. We show that a membrane is necessary for the conformational conversion because membrane protein interactions counteract the loss of the many intra-protein hydrophobic interactions that stabilize the membrane-inserting segments in the Clya monomer. Of the two membrane-inserting segments, the flexible and highly hydrophobic β-tongue inserts into the membrane, while the insertion of Helix A1 is membrane assisted. We conclude that the β-tongue is designed to behave as a rapid membrane sensor, while Helix A1 improves target selectivity for cholesterol containing cell membranes. We connect these results with recent single molecule experiments carried out in our laboratory and comment on the implications of the study on the pore forming pathway for ClyA.