In Escherichia coli, secretory proteins (preproteins) are translocated across the cytoplasmic membrane by the Sec system composed of a protein-conducting channel, SecYEG, and an ATP-dependent motor protein, SecA. After binding of the preprotein to SecYEG-bound SecA, cycles of ATP binding and hydrolysis by SecA are thought to drive the stepwise translocation of the preprotein across the membrane. To address how the length of a preprotein substrate affects the SecA-driven translocation process, we constructed derivatives of the precursor of the outer membrane protein A (proOmpA) with 2, 4, 6, and 8 in-tandem repeats of the periplasmic domain. With increasing polypeptide length, an increasing delay in the time before full-length translocation was observed, but the translocation rate expressed as amino acid translocation per minute remained constant. These data indicate that in the ATP-dependent reaction, SecA drives a constant rate of preprotein translocation consistent with a stepping mechanism of translocation.

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Journal J. Biol. Chem.
Tomkiewicz, D, Nouwen, N, van Leeuwen, R.G.H, Tans, S.J, & Driessen, A.J.M. (2006). SecA supports a constant rate of preprotein translocation. J. Biol. Chem., 281, 15709–15713. doi:10.1074/jbc.m600205200