Precursor Proteins Are Intermediates in vivo, in the Synthesis of Two Major Outer Membrane Proteins, the OmpA and OmpF Proteins, of Escherichia coli K12

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Abstract
The OmpA and OmpF proteins are major outer membrane proteins ofEscherichia coli K12. Their precursors, the pro‐OmpA and pro‐OmpF proteins, have been detected in vivo in pulse‐labelling experiments carried out with [35S]methionine at 25°C. When the pulse was at 37°C, however, no precursors were detected. The pulse‐labelled precursors were processed rapidly and quantitatively into mature protein at 25°C. The apparent half‐life of the pro‐OmpF protein was estimated to be 30 s, and the pro‐OmpA protein may be processed even faster. In short pulses (10 s) the precursors of both proteins were the predominant labelled species, indicating that at 25°C processing does not start until chain elongation of the precursor is almost, if not entirely, complete. When French press lysates of cells pulse‐labelled for 10 s were subjected to sucrose gradient centrifugation to separate the inner and outer membranes, both precursors comigrated with the inner membrane.