Signal recognition particle-dependent insertion of coronavirus E1, an intracellular membrane glycoprotein

The membrane insertion of the E1 protein of a coronavirus, mouse hepatitis virus A59, was studied in a wheat germ cell-free translation system. E1 is a transmembrane protein spanning the lipid bilayer several times. It is synthesized without a cleavable signal sequence, localized intracellularly, and not transported to the cell surface. It thus represents a model intracellular protein. We found that the synthesis of E1 is specifically and stably blocked by the addition of signal recognition particle to the wheat germ system. Subsequent addition of salt-extracted pancreatic microsomes resulted in the full release of this arrest as well as the completion and the correct membrane integration of E1. Such signal recognition particle-induced arrests failed to produce shorter peptides of a defined length. Addition of signal recognition particle to a synchronized translation at any time during the synthesis of about the first two thirds of E1 (150 amino acids) blocked further translation, suggesting that the most C-terminal of the three internal hydrophobic domains of E1 could function as its signal sequence.