Cholesterol-induced protein sorting: an analysis of energetic feasibility

Abstract

The mechanism(s) underlying the sorting of integral membrane proteins between the Golgi complex and the plasma membrane remain uncertain because no specific Golgi retention signal has been found. Moreover one can alter a protein's eventual localization simply by altering the length of its transmembrane domain (TMD). M. S. Bretscher and S. Munro (SCIENCE: 261:1280-1281, 1993) therefore proposed a physical sorting mechanism based on the hydrophobic match between the proteins' TMD and the bilayer thickness, in which cholesterol would regulate protein sorting by increasing the lipid bilayer thickness. In this model, Golgi proteins with short TMDs would be excluded from cholesterol-enriched domains (lipid rafts) that are incorporated into transport vesicles destined for the plasma membrane. Although attractive, this model remains unproven. We therefore evaluated the energetic feasibility of a cholesterol-dependent sorting process using the theory of elastic liquid crystal deformations. We show that the distribution of proteins between cholesterol-enriched and cholesterol-poor bilayer domains can be regulated by cholesterol-induced changes in the bilayer physical properties. Changes in bilayer thickness per se, however, have only a modest effect on sorting; the major effect arises because cholesterol changes also the bilayer material properties, which augments the energetic penalty for incorporating short TMDs into cholesterol-enriched domains. We conclude that cholesterol-induced changes in the bilayer physical properties allow for effective and accurate sorting which will be important generally for protein partitioning between different membrane domains.

FIGURE 1

(A) Lateral sorting of membrane proteins (dark-hatched) between thin, cholesterol-poor bilayer domains (light gray) and thicker, cholesterol-enriched bilayer domains (cross-hatched). The proteins will tend toward the domain in which there is hydrophobic match between the protein length and the bilayer thickness. (B) In a nondeformable lipid bilayer, a mismatch between the hydrophobic thickness of the bilayer and the protein hydrophobic length leads to exposure of hydrophobic surface to the aqueous surroundings. (C) In a deformable bilayer, the hydrophobic coupling between the protein and the bilayer induces a bilayer deformation.

FIGURE 2

ΔG_def of inserting α-helices having 15–20 AA into SOPC (•); SOPC:Chol (1:1) bilayers (▪); and a bilayer with a thickness corresponding to SOPC:Chol (1:1) but with material properties as SOPC (□).

FIGURE 3

(A) Effects of cholesterol on the material moduli of SOPC bilayers having various f_Chol. (•) K_a measured by Needham and Nunn (1990); (▾) K_c calculated using Eq. 3. (B) The effect of cholesterol on ΔG_def for α-helices having 15–20 AA.

FIGURE 4

(A, C) Effects of cholesterol on the lateral partition coefficient, K_SOPC:Chol, of α-helices (A), and multihelical membrane proteins (C), of different length, between SOPC and SOPC:Chol bilayer domains. (B, D) Effects on the partition coefficient of α-helices (B), and multihelical membrane proteins (D), of different length, between SOPC and a bilayer domain with a thickness corresponding to SOPC:Chol, but with material properties as SOPC.