Exchange of water for sterol underlies sterol egress from a StARkin domain.

George Khelashvili,David Eliezer,Kalpana Pandey,Neha Chauhan,Anant K Menon

doi:10.7554/elife.53444

Abstract

Previously we identified Lam/GramD1 proteins, a family of endoplasmic reticulum membrane proteins with sterol-binding StARkin domains that are implicated in intracellular sterol homeostasis. Here, we show how these proteins exchange sterol molecules with membranes. An aperture at one end of the StARkin domain enables sterol to enter/exit the binding pocket. Strikingly, the wall of the pocket is longitudinally fractured, exposing bound sterol to solvent. Large-scale atomistic molecular dynamics simulations reveal that sterol egress involves widening of the fracture, penetration of water into the cavity, and consequent destabilization of the bound sterol. The simulations identify polar residues along the fracture that are important for sterol release. Their replacement with alanine affects the ability of the StARkin domain to bind sterol, catalyze inter-vesicular sterol exchange and alleviate the nystatin-sensitivity of lam2Δ yeast cells. These data suggest an unprecedented, water-controlled mechanism of sterol discharge from a StARkin domain.

Highlights

Cholesterol, the 'central lipid of mammalian cells' (1), is the most abundant molecular component of the mammalian plasma membrane (PM) where it represents one out of every 23 lipids (1,2)
Lam1-Lam4 localize to endoplasmic reticulum (ER)-PM contact sites in yeast (7,15) where they play a role in sterol homeostasis
We used atomistic molecular dynamics (MD) simulations of Lam4S2 to explore the impact of the unique lateral opening in Lam/GramD1 StARkin domains on the stability of bound sterol and its ability to exit the binding pocket

Summary

Introduction

Cholesterol, the 'central lipid of mammalian cells' (1), is the most abundant molecular component of the mammalian plasma membrane (PM) where it represents one out of every 23 lipids (1,2). Proteins with steroidogenic acute regulatory protein related lipid transfer (StART) domains constitute a major family of intracellular lipid transport proteins - the StARkin superfamily - implicated in moving glycerophospholipids, ceramide and sterol between cellular membranes (5,6). Whereas these proteins are generally soluble and able to diffuse freely through the cytoplasm, a new family of ER membrane proteins with StARkin domains was recently identified, including six members (Lam1-Lam6) in the budding yeast Saccharomyces cerevisiae and three members (GramD1aGramD1c) in mammals (7-10). A sterol homeostatic role has been suggested for mouse

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Conclusion