The fibrinogen γ-module sequences, γ190−202 or P1, and γ377−395 or P2, were implicated in interaction with the αMI-domain of the leukocyte receptor αMβ2. P1 is an integral part of the γ-module central domain, while P2 is inserted into this domain forming an antiparallel β-strand with P1. We hypothesized earlier that separation of P2 from P1 may regulate interaction of fibrin(ogen) with leukocytes during the inflammatory response. To test the relative contributions of these sequences to the interaction and the effect of their separation, we prepared the recombinant γ-module (γ148−411) and its halves, γ148−286 and γ287−411 fragments containing P1 and P2, respectively, and evaluated their affinities for the recombinant αMI-domain. In a solid-phase binding assay, the immobilized γ-module exhibited high affinity for αMI (Kd = 22 nM), while the affinities of the isolated γ148−286 and γ287−411 halves were much lower (Kd's = 521 and 194 nM, respectively), indicating that both halves contribute to the interaction ...