Studies on the export of the maltose-binding protein and the LamB protein

Two proteins encoded in the malB locus, the maltose-binding protein and the LamB protein, are exported to the periplasm and outer membrane, respectively. Both proteins are synthesized on membrane-bound polysomes in precursor forms which contain signal sequences at their amino-termini. Proteolytic removal of the signal sequence from maltose-binding protein begins co-translationally but is nat initiated until the nascent chains have attained 80% of their final length. Approximately one-third of the molecules are processed co-translationally, the remainder being processed post-translationally. In contrast, the LamB precursor is processed almost entirely post-translationally. In neither case is it clear when the polypeptide chains are transferred through the cytoplasmic membrane. An interesting feature with as yet unknown significance is the non-uniform rate of elongation of maltose-binding protein. The export of maltose-binding protein and the lamB protein as well as that of other proteins, has been shown to require an energized membrane.