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Volume 271, Number 24, Issue of June 14, 1996 pp. 14143-14149
©1996 by The American Society for Biochemistry and Molecular Biology, Inc.

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Membrane Insertion Characteristics of the Various Transmembrane Domains of the Escherichia coli TolQ Protein

(Received for publication, November 3, 1995, and in revised form, March 26, 1996)

From the Department of Biochemistry, Duke University Medical Center, Durham, North Carolina 27710

The Escherichia coli TolQ protein is a 230-amino acid integral cytoplasmic membrane protein required for the import of group A colicins, for infection by the filamentous phage, and for maintenance of the integrity of the bacterial envelope. TolQ is a polytopic protein with three membrane-spanning regions. The first membrane-spanning region has a 19-residue periplasmic NH₂-terminal tail, while the second and third membrane-spanning segments are separated by a short 17-amino acid periplasmic loop. To study the membrane assembly of TolQ, fusions of different membrane-spanning regions were examined for their ability to insert in the absence of functional SecA or the membrane potential. Fusions containing the first membrane-spanning region plus the adjacent cytoplasmic domain and a construct containing the ``hairpin loop,'' formed by the second and third membrane-spanning regions, insert in the absence of functional SecA. The fusion containing the second and third membrane-spanning regions required the membrane potential for insertion while the first membrane-spanning region was able to insert even in the absence of a membrane potential. Taken together, these results suggest that insertion of intact TolQ is not dependent on the Sec system, but does require the membrane potential.

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