LOCAL FOLDING OF THE N-TERMINAL DOMAIN OF ESCHERICHIA-COLI RECA CONTROLS PROTEIN-PROTEIN INTERACTION

To obtain structural information about the self-association of the pro tein RecA, we studied urea denaturation of RecA by circular dichroism spectroscopy and gelfiltration. Gel filtration analysis showed that ur ea at low concentrations, 1.0-1.2 M, dissociated the RecA oligomer to almost a monomeric state prior to the unfolding of each molecule, Upon treatment with 1.0 M urea, the circular dichroism spectrum showed a d ecrease in the a-helical content of RecA, A similar decrease was obser ved in the absence of urea for RecA at an extremely low protein concen tration; the RecA oligomer dissociated to an almost completely monomer ic state, The properties of RecA at low urea concentrations were simil ar to those of a truncated RecA lacking the first 33 N-terminal residu es (Delta 33RecA). Addition of a synthetic peptide corresponding to th e 33 N-terminal residues to Delta 33RecA increased the alpha-helical c ontent, These results suggest that local folding of the N-terminal dom ain is coupled to protein-protein interactions of monomeric RecA, whic h are involved in the regulation of filament formation, The dissociati on constant for interaction between RecA monomers was determined from the ellipticity data to be 0.1 mu M.