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J. Biol. Chem., Vol. 278, Issue 37, 35265-35271, September 12, 2003

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Disulfide Cross-linking Reveals a Site of Stable Interaction between C-terminal Regulatory Domains of the Two MalK Subunits in the Maltose Transport Complex^*

Susmita Samanta  , Tulin Ayvaz , Moriama Reyes ¶, Howard A. Shuman ¶, Jue Chen || and Amy L. Davidson  **

From the Department of Molecular Virology and Microbiology, Baylor College of Medicine, Houston, Texas 77030, ^¶Department of Microbiology, College of Physicians & Surgeons, Columbia University, New York, New York 10032, and ^||Department of Biological Sciences, Purdue University, West Lafayette, Indiana 47907

Understanding the structure and function of the ATP-binding cassette (ABC) transporters is very important because defects in ABC transporters lie at the root of several serious diseases including cystic fibrosis. MalK, the ATP-binding cassette of the maltose transporter of Escherichia coli, is distinct from most other ATP-binding cassettes in that it contains an additional C-terminal regulatory domain. The published structure of a MalK dimer is elongated with C-terminal domains at opposite poles (Diederichs, K., Diez, J., Greller, G., Muller, C., Breed, J., Schnell, C., Vonrhein, C., Boos, W., and Welte, W. (2000) EMBO J. 19, 5951–5961). Some uncertainty exists as to whether the orientation of MalK in the dimer structure is correct. Superpositioning of the N-terminal domains of MalK onto the ATP-binding domains of an alternate ABC dimer, in which ATP is bound along the dimer interface between Walker A and LSGGQ motifs, places both N- and C-terminal domains of MalK along the dimer interface. Consistent with this model, a cysteine substitution at position 313 in the C-terminal domain of an otherwise cysteine-free MalK triggered disulfide bond formation between two MalK subunits in an intact maltose transporter. Disulfide bond formation did not inhibit the function of the transporter, suggesting that the C-terminal domains of MalK remain in close proximity throughout the transport cycle. Enzyme IIA^glc still inhibited the ATPase activity of the disulfide-linked transporter indicating that the mechanism of inducer exclusion was unaffected. These data support a model for ATP hydrolysis in which the C-terminal domains of MalK remain in contact whereas the N-terminal domains of MalK open and close to allow nucleotide binding and dissociation.

Received for publication, February 3, 2003 , and in revised form, May 29, 2003.

^* This work was supported by National Institutes of Health Grant GM49261 and Grant Q-1391 from the Robert A. Welch Foundation. 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: Dept. of Molecular Genetics, The University of Texas, M. D. Anderson Cancer Center, 1515 Holcombe Blvd. Houston, TX 77030.

^** To whom correspondence should be addressed: Dept. of Molecular Virology and Microbiology, Baylor College of Medicine, One Baylor Plaza, MS: BCM-280, Houston, TX 77030. Tel.: 713-798-4552; Fax: 713-798-7375; E-mail: davidson{at}bcm.tmc.edu.

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