Synthesis and assembly of the membrane proteins in E. coli.

Kinetics of integration of membrane proteins were studied in E. coli to discover how membrane proteins find their final location in the functional membrane. The experiments make use of a simple and convenient method developed for isolating inner and outer membranes from a number of small-scale cultures with high recovery. Among the proteins that constitute the cell surface structures, inner membrane proteins are integrated most rapidly after synthesis, whereas outer membrane proteins delay somewhat, and periplasmic proteins delay further in reaching their destinations. Protein I, a major outer membrane protein with molecular weight of about 37,000 daltons, exhibits significantly slower rates of integration than other outer membrane proteins. The decreased fluidity of membrane lipids by temperature shiftdown of an unsaturated fatty acid auxotroph grown on elaidate results in abnormally slow assembly of the outer membrane proteins and also in an anomalous assembly of the inner membrane proteins, suggesting that the fluid state of the lipids is required for normal operation of these processes. The possible relevance of these findings to the mechanism of membrane formation is discussed.