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J. Biol. Chem., Vol. 279, Issue 32, 33290-33297, August 6, 2004

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Evidence for Multiple Pathways in the Assembly of the Escherichia coli Maltose Transport Complex^*

Kathleen A. Kennedy, Eliora G. Gachelet, and Beth Traxler

From the Department of Microbiology, University of Washington, Seattle, Washington 98195

We used the maltose transport complex MalFGK₂ of the Escherichia coli cytoplasmic membrane as a model for the study of the assembly of hetero-oligomeric membrane protein complexes. Analysis of other membrane protein complexes has led to a general model in which a unique, ordered pathway is followed from subunit monomers to a final oligomeric structure. In contrast, the studies reported here point to a fundamentally different mode for assembly of this transporter. Using co-immunoprecipitation and quantification of interacting partners, we found that all subunits of the maltose transport complex efficiently form heteromeric complexes in vivo. The pairwise complexes were stable over time, suggesting that they all represent assembly intermediates for the final MalFGK₂ transporter. These results indicate that several paths can lead to assembly of this oligomer. We also characterized MalF and MalG mutants that caused reduced association between some or all of the subunits of the complex with this assay. The mutant analysis highlights some important motifs for subunit contacts and suggests that the promiscuous interactions between these Mal proteins contribute to the efficiency of complex assembly. The behaviors of the wild type and mutant proteins in the co-immunoprecipitations support a model of multiple assembly pathways for this complex.

Received for publication, April 5, 2004 , and in revised form, June 7, 2004.

^* This research was supported by Grant RPG-98-223-01-CSM from the American Cancer Society. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

Present address: Institute for Systems Biology, Institute for Systems Biology, 1441 N. 34th St., Seattle, WA 98103-8904.

To whom correspondence should be addressed: University of Washington, Department of Microbiology, Box 357242, Seattle, WA 98195. Tel.: 206-543-5485; Fax: 206-543-8297; E-mail: btraxler{at}u.washington.edu.

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