Lipid fluidity-dependent biosynthesis and assembly of the outer membrane proteins of E. coli.

The reduction of the membrane lipids of E. coli to a nonfluid state resulted in the accumulation in the cell envelope of a high molecular weight precursor of the protoIG protein, a major outer membrane protein. The protoIG protein was as sensitive to trypsin as the mature toIG protein assembled in the outer membrane. In contrast to the toIG protein, however, the accumulated protoIG protein was easily released from the envelope fraction by both sodium lauryl sarcosinate extraction and sonication. This indicated that the precursor protein was loosely associated with the cell membrane. When a fluid lipid state was restored, the protoIG protein was processed to the mature form which was then correctly assembled in the outer membrane. These results suggest that the protoIG protein produced under nonfluid lipid conditions was properly translocated across the cytoplasmic membrane, but could not be assembled in the outer membrane due either to the reversible inhibition of the processing of the ProtoIG to the toIG protein or to the lack of interaction with a specific outer membrane component(s). Reduced lipid fluidity also caused various alterations in the biosynthesis and assembly of other membrane proteins. In addition to the toIG protein, a large number of new proteins were accumulated in the membrane. Alternatively, the matrix protein as well as the promatrix protein were not detected in the cell envelope. On the other hand, the lipoprotein was normally produced, processed, modified and assembled in the outer membrane. These results indicate that the outer membrane proteins are synthesized and assembled according to several different mechanisms, on which the physical state of the membrane has various effects.