Monomer-tetramer equilibrium of the Escherichia coli ssb-1 mutant single strand binding protein

The Escherichia coli single strand binding (SSB) protein is an essential protein required for DNA replication and involved in recombination and a number of repair processes. It is a stable homotetramer in solution; however the ssb-1 mutation (His-55 to Tyr) destabilizes the tetramer with respect to monomers and this defect seems to explain the observed phenotype (Williams, K. R., Murphy, J. B., and Chase, J. W. (1984) J. Biol. Chem. 259, 11804-11811). We report a quantitative study of the SSB-1 monomer-tetramer equilibrium in vitro as a function of temperature, pH, NaCl, MgCl₂, urea, and guanidine hydrochloride concentrations. The self-assembly equilibrium was monitored by the increase in intrinsic protein fluorescence anisotropy accompanying the formation of the tetramer. The experimental isotherms indicate that SSB-1 dimers are not highly populated at equilibrium, hence the formation of the tetramer is well-described as a one-step association of four monomers. At 25°C, pH 8.1, the monomer concentration for 50% tetramer dissociation is (M_T)_1/2 = 0.87 μM, corresponding to a monomer-tetramer equilibrium constant, K_T = 3 ± 1 × 10¹⁸ M^-3. The tetramerization constant, K_T, is highly dependent upon temperature and pH, with ΔH°= -51 ± 7 kcal/mol (pH 8.1) and ΔH° = -37 ± 5 kcal/mol (pH 6.9). There is no effect of NaCl on the monomer-tetramer association in the range from 0.20 to 1.0 M; however, MgCl₂ decreases the stability of the SSB-1 tetramer. In the presence of high concentrations of the single-stranded oligonucleotide, dT(pT)₁₅, the tetramerization constant is slightly increased indicating that binding of the oligonucleotide to the SSB-1 monomer promotes the assembly process, although not dramatically. The large negative ΔH° that is associated with formation of the tetramer provides a likely explanation for the temperature sensitivity of the ssb-1 mutation.