Membrane biogenesis. Evidence that a soluble chimeric polypeptide can serve as a precursor of a mutant lac permease in Escherichia coli.

A mutant in the Escherichia coli lac permease, called Yf, appears to be defective in the biogenesis and proper assembly of this membrane protein. It was proposed that this defect led to the accumulation of a precursor of the mutant permease (Fried, V. A. (1977) J. Mol Biol. 114, 477-490). In this communication, evidence is presented that the lacYf mutant accumulates a novel lac-specific soluble polypeptide with a molecular weight of 87,000. Detected by double-label analysis on sodium dodecyl sulfate gels, and identified as a lac-specific polypeptide on a two-dimensional gel system, this polypeptide is immunoprecipitated by anti-transacetylase antibody. Pulse-chase experiments are consistent with the hypothesis that it is converted in vivo into a lac-specific membrane protein with an apparent molecular weight of 28,000, which appears to be the mutant lac permease. The results suggest that the 87,000-dalton soluble protein is a precursor of the mutant lac permease. It is proposed that this precursor is a polyprotein chimera containing both the lacY and lacA gene products.