Membrane orientation of rat gp110 as studied by in vitro translation

We have recently isolated a full-length cDNA clone for a membrane glycoprotein with apparent Mr of 110,000, denoted gp110, that is expressed in rat liver, kidney, small intestine, and lung. Analysis of the amino acid sequence derived from cDNA sequencing demonstrated that there is a highly hydrophobic domain at the amino terminus (amino acid residues 1-29) that looks like the amino-terminal signal sequence. Using in vitro transcription and translation systems, we have expressed the full-length gp110 cDNA transcript, as well as transcripts derived from truncated gp110 cDNA that terminate translation at different sites. As expected, the amino-terminal signal sequence can promote the translocation across microsomal membranes of the downstream sequence. Like most translocations, the cotranslational translocation of the downstream sequence initiated by the gp110 amino-terminal signal sequence is mediated by signal recognition particle and docking protein. Unlike most amino-terminal signal sequences of lysosomal, secretory, and membrane proteins, the amino-terminal signal sequence of gp110 is not cleaved and may be involved in anchoring the polypeptide to the membrane. We, therefore, predict that the membrane orientation of gp110 is of type II with an extremely small amino-terminal cytoplasmic domain (6 residues).