Signal recognition particle mediates the insertion of a transmembrane protein which has a cytoplasmic NH2 terminus

The mechanism by which rat liver asialoglycoprotein receptor (rat hepatic lectin, RHL) is inserted into membranes has been investigated. RHL is a prototype for transmembrane proteins which are oriented with their NH2 termini in the cytoplasm and their COOH termini outside the cell. Such transmembrane proteins are synthesized without cleavable NH2-terminal signal sequences. An in vitro translation system has been developed in which RHL is translated from RNA produced in a bacteriophage SP6 in vitro transcription system. RHL produced in this way can be inserted cotranslationally in the correct orientation into dog pancreas microsomes. This insertion process has been shown to be dependent on the signal recognition particle and its receptor (the docking protein) in the microsomal membranes. A detailed mechanism for the insertion of this type of transmembrane protein into the lipid bilayer is proposed.