Diagram of the proposed molecular interactions of Pg with cell surface receptors. (A) In cell suspension, soluble Pg (black) binds lysines (Lys) via its LBS. Integrin α₅β₁ is involved in Pg activation presumably by binding soluble Pg. (All cell receptors are in gray.) (B) An adherent cell spreads on Pg bound to the ECM. Unfolded immobilized Pg interacts with all 3 cell surface receptors.