A mutant, in which a cysteine has been site-directed into the polar loop region of the c subunit at residue 44, has been studied. Cross-linking of the c subunit to both the γ and ϵ subunits was observed with cupric 1,10-phenanthrolinate treatment. The linkage between the c and γ subunits was localized to that part of the γ subunit between residues 202–286, based on peptide analysis. Reference to the high resolution structure of F1 [Abrahams et al. (1994) Nature 370, 621–628] appears to limit this contact site to the region including residues 202–230. This segment contains 4 tyrosines and 1 tryptophan as possible reactive residues for cross-linking with the c subunit cysteine.
