Location of filamentous phage minor coat proteins in phage and in infected cells.

We show that all minor coat proteins of phage f1 are integral inner membrane proteins prior to assembly. Hence all phage structural and morphogenetic proteins are localized in the membrane of the infected cell, consistent with models of phage assembly in which morphogenesis is coincident with phage extrusion. Our data suggest that the minor coat proteins, pVI and pIII, are already associated with the major coat protein, pVIII, in the membrane. On the other hand pVI and pIII are not recovered as a complex from the membrane, even though experiments with dissociated phage show they are associated in phage. The minor coat proteins, pVII and pVIII, are also associated in phage. With the use of sera directed against the minor proteins, we show that the minor coat protein, pIX, is accessible in intact phage, but pVI and pVII are not. Consistent with earlier results, the attachment protein, pIII, clearly is exposed on the phage exterior. Infected cells contain about ten times more pVII and pIX than is incorporated into phage.