An artificial transmembrane segment directs SecA, SecB, and electrochemical potential-dependent translocation of a long amino-terminal tail.
Many integral membrane proteins contain an amino-terminal segment, often referred to as an N-tail, that is translocated across a membrane. In many cases, translocation of the N-tail is initiated by a cleavable, amino-terminal signal peptide. For N-tail proteins lacking a signal peptide, translocation is initiated by a transmembrane segment that is carboxyl to the translocated segment. The mechanism of membrane translocation of these segments, although poorly understood, has been reported to be independent of the protein secretion machinery. In contrast, here we describe alkaline phosphatase mutants containing artificial transmembrane segments that demonstrate that translocation of a long N-tail across the membrane is dependent upon SecA, SecB, and the electrochemical potential in the absence of a signal peptide. The corresponding mutants containing signal peptides also use the secretion machinery but are less sensitive to inhibition of its components. We present evidence that inhibition of SecA by sodium azide is incomplete even at high concentrations of inhibitor, which suggests why SecA-dependent translocation may not have been detected in other systems. Furthermore, by varying the charge around the transmembrane segment, we find that in the absence of a signal peptide, the orientation of the membrane-bound alkaline phosphatase is dictated by the positive inside rule. However, the presence of a signal peptide is an overriding factor in membrane orientation and renders all mutants in an Nout-Cin orientation.
Department of Molecular and Cell Biology, The University of Connecticut, Storrs, Connecticut 06269, USA.
J L
JL
McMurry
D A
DA
Kendall
D000251:Adenosine Triphosphatases / Q000737:chemistry* / Q000378:metabolism; D000595:Amino Acid Sequence; D001426:Bacterial Proteins / Q000737:chemistry* / Q000378:metabolism; D001692:Biological Transport; D002462:Cell Membrane / Q000737:chemistry* / Q000378:metabolism; D004926:Escherichia coli / Q000378:metabolism; D029968:Escherichia coli Proteins; D026901:Membrane Transport Proteins; D008969:Molecular Sequence Data; D000069816:SEC Translocation Channels; D000081416:SecA Proteins; D055672:Static Electricity
D001426:Bacterial Proteins; D029968:Escherichia coli Proteins; D026901:Membrane Transport Proteins; D000069816:SEC Translocation Channels; C057842:SecB protein, Bacteria; D000251:Adenosine Triphosphatases; D000081416:SecA Proteins