The human glucose transporter can insert posttranslationally into microsomes

Abstract

RNA transcripts encoding the complete human glucose transporter (GT) or fragments of GT corresponding to the NH 2-terminal 340 amino acids (GT-N) or the COOH-terminal 148 amino acids (GT-C) were synthesized in vitro from SP6 plasmids. The corresponding polypeptides were synthesized in reticulocyte or wheat germ cell-free systems and their insertion into pancreatic microsomes was assessed by endoglycosidase H and trypsin digestion or alkaline extraction of membranes. The following observations were made: both GT and GT-N can insert posttranslationally into microsomes; GT contains at least two distinct signal sequences; both co- and posttranslational insertion of GT-N are mediated by a signal recognition particle (SRP)-SRP receptor-dependent mechanism; and SRP is required both for targeting the polypeptide to the membrane and for initiating the actual insertion process. We propose a model for the biosynthesis of GT and proteins of similar membrane topology which does not require a tight ribosome-membrane interaction.