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J Biol Chem, Vol. 274, Issue 27, 19397-19402, July 2, 1999

Mutational Alterations in the Homotetrameric Chaperone SecB That Implicate the Structure as Dimer of Dimers

Eva M. Murén, Dominic Suciu, Traci B. Topping, Carol A. Kumamoto^**, and Linda L. Randall

From the  Department of Biochemistry and Biophysics, Washington State University, Pullman, Washington 99164-4660 and the ^** Department of Molecular Biology and Microbiology, Tufts University School of Medicine, Boston, Massachusetts 02111

Variant forms of SecB with substitutions of aminoacyl residues in the region from 74 to 80 were analyzed with respect to their ability to bind a physiological ligand, precursor galactose-binding protein, and to their oligomeric states. SecBL75Q and SecBE77K are tetramers with affinity for ligand indistinguishable from that of the wild-type SecB, and thus the export defect exhibited by strains producing these variants must result from an effect on interactions between SecB and other components. SecBF74I is tetrameric but binds ligand with a lower affinity. Substitutions at positions 76, 78, and 80 cause a shift in the equilibrium so that the SecB tetramer dissociates into dimers. We conclude that the tetramer is a dimer of dimers and that the residues Cys⁷⁶, Val⁷⁸, and Gln⁸⁰ must be involved either directly or indirectly in forming the interface between dimers. These variant species are defective in binding ligand; however, because their oligomeric state is altered no conclusion can be drawn concerning the direct role of these residues in ligand binding.

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