Protein Gymnastics in Thelipid Bilayer: Lipides as Determinants of Protein Structure

Abstract

The orientation of transmembrane domains of polytopic membrane proteins with respect to the plane of the lipid bilayer is determined by a complex interplay between topogenic signals residing within the protein sequence, interaction of the protein with the translocon membrane insertion machinery, short-range and long-range interactions within the protein and the final environment of the protein largely determined by membrane lipid composition. Systematic alteration of lipid composition at steady state or dynamically after membrane protein assembly uncovered a role for lipid-protein interactions in determining initial membrane protein topogenesis as well as in dynamic topological re-organization after initial protein folding. Alteration of the charges within protein extramembrane domains coupled with changes in lipid composition demonstrated a synergistic and reciprocal relationship between protein and lipid charges in determining orientation of transmembrane domains. These results led to the Charge Balance Hypothesis, which posits that protein topogenic signals are decoded in accordance with positive-inside rule initially by the translocon but finally interpreted by electrostatic interactions between protein extramembrane domains and the membrane surface charge as determined by the collective membrane lipid head group composition. The steady state and dynamic nature of membrane protein organization observed in whole cells, as a function of the Charge Balance Hypothesis, has been faithfully reproduced with membrane proteins reconstituted in proteoliposomes thus establishing that such initial and dynamic protein organization is dependent solely on direct lipid-protein interactions independent of other cellular factors. Therefore, membrane protein topological organization can be viewed as highly dynamic rather than stable and static. The dynamic view of protein topological organization as influenced by the membrane lipid environment reveals previously unrecognized possibilities for cellular regulation and understanding of disease states resulting from mis-folded membrane proteins. Supported in part by NIH grant R37-GM20478.