title: Translocation of domains of nascent periplasmic proteins across the cytoplasmic membrane is independent of elongation.
abstract: Accessibility of nascent chains of periplasmic proteins to externally added proteinase K was used as the criterion for translocation of polypeptides across the cytoplasmic membrane of E. coli during the process of export. It is concluded for maltose-binding protein and ribose-binding protein that nascent chains carrying the signal sequence are not accessible to the proteinase while chains that have been matured span the membrane and are degraded. Translocation of polypeptides is a late event relative to extent of elongation, occurring only after maltose-binding protein has reached molecular weight 33,000 (80% of its entire length) and after ribose-binding protein has been fully elongated (molecular weight 29,000). The data presented here are inconsistent with postulated mechanisms of export requiring a strict coupling of translocation to elongation of nascent polypeptide chains. In contrast, the data support the idea that entire domains of polypeptides are transferred after their synthesis. This is the case whether the translocation of a protein is initiated post-translationally or begins before synthesis of the entire protein is completed.
mesh_terms: D018528:ATP-Binding Cassette Transporters; D001426:Bacterial Proteins / Q000096:biosynthesis / Q000378:metabolism*; D001692:Biological Transport; D002352:Carrier Proteins / Q000378:metabolism; D002462:Cell Membrane / Q000378:metabolism; D019286:Endopeptidase K; D010450:Endopeptidases; D004926:Escherichia coli / Q000378:metabolism*; D029968:Escherichia coli Proteins; D058619:Maltose-Binding Proteins; D009004:Monosaccharide Transport Proteins; D033902:Periplasmic Binding Proteins; D011499:Protein Processing, Post-Translational
chemical_list: D018528:ATP-Binding Cassette Transporters; D001426:Bacterial Proteins; D002352:Carrier Proteins; D029968:Escherichia coli Proteins; C461619:MalE protein, E coli; D058619:Maltose-Binding Proteins; D009004:Monosaccharide Transport Proteins; D033902:Periplasmic Binding Proteins; C028003:RbsB protein, E coli; C461599:maltose transport system, E coli; D010450:Endopeptidases; D019286:Endopeptidase K