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Volume 271, Number 47, Issue of November 22, 1996 pp. 29698-29706
©1996 by The American Society for Biochemistry and Molecular Biology, Inc.

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A Significant Fraction of Functional SecA Is Permanently Embedded in the Membrane
SecA CYCLING ON AND OFF THE MEMBRANE IS NOT ESSENTIAL DURING PROTEIN TRANSLOCATION

(Received for publication, June 24, 1996)

From the Department of Biology, Georgia State University, Atlanta, Georgia 30303

SecA has been suggested to cycle on and off the cytoplasmic membrane of Escherichia coli during protein translocation. We have reconstituted ³⁵S-SecA onto SecA-depleted membrane vesicles and followed the fate of the membrane-associated ³⁵S-SecA during protein translocation. Some ³⁵S-SecA was released from the membranes in a translocation-independent manner. However, a significant fraction of ³⁵S-SecA remained on the membranes even after incubation with excess SecA. This fraction of ³⁵S-SecA was shown to be integrated into the membrane and was active in protein translocation, indicating that SecA cycling on and off membrane is not required for protein translocation. Proteolysis experiments did not support the model of SecA insertion and deinsertion during protein translocation; instead, a major 48-kDa domain was found persistently embedded in the membrane regardless of translocation status. Thus, in addition to catalyzing ATP hydrolysis, certain domains of SecA probably play an important structural role in the translocation machinery, perhaps forming part of the protein-conducting channels.

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