Post-translational insertion of fragment of the glucose transporter into microsomes requires phosphoanhydride bond cleavage

Abstract

Most eukaryotic secretory and membrane proteins insert co-translationally into the membrane of the rough endoplasmic reticulum (RER), and are targeted there by one or more NH₂-terminal or internal signal sequences (for reviews see refs 1,2). However, little is known about the actual translocation and membrane integration processes. In particular, any energy requirements for targeting and integration have remained obscure because of the inability to uncouple the processes from concomitant protein synthesis. We recently showed that the human glucose transporter (GT), an integral membrane glycoprotein³, can insert post-translationally into dog pancreatic microsomes with low but demonstrable efficiency in vitro, and that a fragment corresponding to the NH₂-terminal 340 amino acids and 8 of the 12 membrane-spanning α-helixes of GT (GT-N) can insert with significantly greater efficiency⁴. We report here that post-translational insertion of GT-N into pancreatic microsomes requires energy in the form of a phosphodiester bond, and suggest that co-translational insertion of proteins into the RER may also require energy independent of that used for polypeptide synthesis.