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Volume 272, Number 13, Issue of March 28, 1997 pp. 8686-8694
©1997 by The American Society for Biochemistry and Molecular Biology, Inc.

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Tetramerization and Single-stranded DNA Binding Properties of Native and Mutated Forms of Murine Mitochondrial Single-stranded DNA-binding Proteins

(Received for publication, October 21, 1996, and in revised form, January 16, 1997)

From the Departments of Internal Medicine and Molecular Biology, University of Texas Southwestern Medical Center, Dallas, Texas 75235-8573

We examined previously unexplored aspects of the tetramerization and single-stranded DNA (ssDNA) binding properties of native, precursor, and mutated forms of mitochondrial ssDNA-binding protein (mtSSB) from a mammalian organism (mouse). Tetramic forms of mtSSB reassemble spontaneously after thermal denaturation and undergo subunit exchange. Binding of mtSSB to ssDNA as a function of protein concentration is nonlinear, suggesting a concentration-dependent transition in intrinsic binding affinity and in the topology of the DNA-protein complex. The cleavable presequence at the amino terminus of the precursor form of mtSSB does not disrupt tetramer formation but has a specific inhibitory effect on DNA binding that is not seen in a fusion protein that substitutes a bulkier peptide moiety in this position. Mutated forms of mtSSB bearing amino acid substitutions at highly conserved amino acid positions exhibit subtle or severe defects in ssDNA binding activity and/or tetramerization, even when assembled into heterotetramers in combination with wild-type mtSSB monomers. These experiments provide new insights into structural and functional properties of mammalian mtSSB and have implications for the pathogenesis of human diseases resulting from defects in mtDNA replication.

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