title: Pleiotropic effects of membrane cholesterol upon translocation of protein across the endoplasmic reticulum membrane.
abstract: Various proteins are translocated through and inserted into the endoplasmic reticulum membrane via translocon channels. The hydrophobic segments of signal sequences initiate translocation, and those on translocating polypeptides interrupt translocation to be inserted into the membrane. Positive charges suppress translocation to regulate the orientation of the signal sequences. Here, we investigated the effect of membrane cholesterol on the translocational behavior of nascent chains in a cell-free system. We found that the three distinct translocation processes were sensitive to membrane cholesterol. Cholesterol inhibited the initiation of translocation by the signal sequence, and the extent of inhibition depended on the signal sequence. Even when initiation was not inhibited, cholesterol impeded the movement of the positively charged residues of the translocating polypeptide chain. In surprising contrast, cholesterol enhanced the translocation of hydrophobic sequences through the translocon. On the basis of these findings, we propose that membrane cholesterol greatly affects partitioning of hydrophobic segments into the membrane and impedes the movement of positive charges.
authors.affiliation: Graduate School of Life Science, University of Hyogo, Kouto 3-2-1, Ako-gun, Hyogo 678-1297, Japan.
authors.forename: Hitoshi
authors.initials: H
authors.lastname: Yamamoto
authors.forename: Hidenobu
authors.initials: H
authors.lastname: Fujita
authors.forename: Yuichiro
authors.initials: Y
authors.lastname: Kida
authors.forename: Masao
authors.initials: M
authors.lastname: Sakaguchi
mesh_terms: D000818:Animals; D002474:Cell-Free System; D002784:Cholesterol / Q000737:chemistry*; D004285:Dogs; D004721:Endoplasmic Reticulum / Q000737:chemistry / Q000378:metabolism*; D036341:Intercellular Signaling Peptides and Proteins / Q000737:chemistry / Q000378:metabolism; D007425:Intracellular Membranes / Q000737:chemistry / Q000378:metabolism*; D008565:Membrane Proteins / Q000037:antagonists & inhibitors / Q000737:chemistry / Q000378:metabolism*; D021382:Protein Sorting Signals / Q000502:physiology; D021381:Protein Transport; D011817:Rabbits; D000069816:SEC Translocation Channels
chemical_list: D036341:Intercellular Signaling Peptides and Proteins; D008565:Membrane Proteins; D021382:Protein Sorting Signals; D000069816:SEC Translocation Channels; D002784:Cholesterol