Membrane biogenesis: cotranslational integration of the bacteriophage f1 coat protein into an Escherichia coli membrane fraction.

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Abstract
The coat protein (CP) of bacteriophage f1 is integrated into an E. coli plasma membrane fraction consisting of inverted vesicles when it is synthesized in a cell-free coupled transcription-translation system supplemented with the inverted vesicles. By using proteolytic enzymes as probes, subsequent peptide mapping and determination of the sequence of the proteolytic products showed that CP was inserted into the inverted vesicles in an orientation indistinguishable from that in inverted vesicles prepared from infected E. coli: only a COOH-terminal portion of .apprxeq. 10 residues was accessible to proteolysis, but the remainder of CP (CP'') was entirely protected. Protection of CP'' was dependent on the integrity of the vesicle membrane, because it was abolished when proteolysis was done in the presence of nonionic detergents. Insertion was observed when the inverted vesicles were present during translation in the cell-free system, not when they were added after translation. The asymmetric insertion of this type of integral membrane protein is strictly coupled to translation. These findings are discussed with respect to prokaryotic membrane biogenesis and are related to bacteriophage f1 assembly and infection.