Processing in vivo of precursor maltose-binding protein in Escherichia coli occurs post-translationally as well as co-translationally.

The mechanism of synthesis of maltose-binding protein (Mr = 38,500), an exported periplasmic protein in Escherichia coli, was investigated in vivo. A precursor to maltose-binding protein (Mr - 41,000), which is identical to the precursor polypeptide synthesized in vitro in a cell-free system, can be detected in vivo indicating that it is not processed to mature size until the polypeptide chain is terminated. The population of incomplete, nascent polypeptide chains of maltose-binding protein was found to contain NH2 termini characteristic of both precursor and mature protein demonstrating that processing occurs co-translationally as well as post-translationally. However, the polypeptide containing the signal sequence must reach a critical size of Mr - 33,000 before any processing takes place.