The coat protein (gene 8 product) of coliphage M13 is an integral protein of the host cell membrane at all stages of virus infection. This protein, when made in a cell-free reaction, has been shown by others to have an additional NH2-terminal peptide region and is referred to as "procoat." It is initially not membrane-bound but, upon exposure to Escherichia coli membrane vesicles or to liposomes prepared from E. coli lipids, it assembles into the bilayer in an integral fashion. Much of this protein is shown to be exposed on the inner surface of the liposome. We suggest that refolding of procoat as it encounters the bilayer is sufficient to transport large segments of the peptide chain through the apolar hydrocarbon core. Membrane proteins share certain structural features that raise fundamental questions about their synthesis and assembly into lipid bilayers. Although many integral membrane proteins possess extensive polar domains on each membrane surface, they are anchored to the bilayer by the interactions of their hydrophobic regions with the hydrocarbon core of the membrane. How do the hydrophilic portions of proteins cross the bilayer en route from their site of synthesis within the cytoplasm to their final place on the cell surface? Many cells grow rapidly, and thus they can assemble new protein into their membranes in a matter of minutes. Each species of protein has a characteristic asymmetric distribution across the plane of the bilayer presumably stabilized by the immeasurably slow rate of protein rotation across the hydrocarbon core, termed "flip-flop." Where is the information that specifies a membrane protein's asymmetric orientation? It has recently been suggested (1) that NH2-terminal "signal" sequences specify the orientation as they interact with specific membranous protein transport systems. Once membrane proteins are extracted from the bilayer and purified in the presence of detergents, they require the continued presence of detergent for their dispersal. How are the hydrophobic regions of these membrane proteins synthesized in the aqueous environment of the cytoplasm without denaturation and aggregation?Previous work (2-5) approached these questions through studies of the small coliphage M13. Seven of the eight M13 virus-coded proteins are membrane-bound (6-8). One of these, the coat protein (gene 8 product), is made in particular abundance by the infected cell (6). During all stages of infection, it spans the cytoplasmic membrane as an integral protein (3, 6). Studies of the synthesis of this protein and its assembly into membranes may help to answer questions about membrane biogenesis.M13 duplex DNA has been used by others (9, 10) to program extracts which support coupled transcription and translation. Analysis by sodium dodecyl sulfate (NaDodSO4)/polyacrylamide gel electrophoresis has shown cell-free synthesis of almost all of the specific M13 proteins. The coat protein synthesized by such extracts has 23 additional residues on its NH2 terminus (10)(11)(12)
Scientific progress often arises from several researchers' synergistic efforts on a particular biological system. One such entity, the major capsid protein of the filamentous coliphage M13 (and fd), has intrigued investigators with the versatility of its structural and functional roles. This article is an attempt to summarize the work of many individuals in a coherent picture of coat protein as it passes through its several natural environments, namely as the major component of the virus coat, as a soluble precursor form in the cytoplasm of infected cells, and as a major transmembrane protein in the infected cell's cytoplasmic membrane. The life cycle of M13, adapted from Kornbergl with an emphasis upon the coat protein, is sketched in FIGURE 1.
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