DOCKING THE MITOCHONDRIAL INHIBITOR PROTEIN-IF(1) TO A MEMBRANE-RECEPTOR DIFFERENT FROM THE F(1)-ATPASE BETA-SUBUNIT

Monoclonal antibodies reacting with the inhibitor protein (IF1) of the mitochondrial ATPase/ATP synthase complex did not modify the IF1-indu ced inhibition of soluble F1 ATPase activity. On the contrary, they in creased the ATPase activity of inverted electron-transport particles w ithout inducing a significant release of IF1 from these particles. Thi s suggested that IF1 could be linked to a membrane protein when it was not inhibiting the ATPase activity. IF1 antibodies have been used to show that IF1 can bind not only to the beta subunit of F1-ATPase [Klei n, G., Satre, M., Dianoux, A. C. & Vignais, P. V. (1981) Biochemistry 20, 1339-1344] but also to a protein present in the inner-mitochondria l membrane. The cross-linking of IF1 to this membrane protein gave a p roduct of M(r) 15000-16000 that migrated differently from IF1 and from the dimer of IF1 using SDS/PAGE. When the cross-linked product was ob tained by using a cleavable cross-linking reagent, the complex between IF1 and the docking protein was partly dissociated and free IF1 was r ecovered. Considering the molecular mass of IF1, this docking protein for IF1 has apparent M(r) 5000-6000. The complex between IF1 and this receptor protein can be detected in low amounts by antibodies against IF1 in the absence of cross-linking reagent. Since this complex remain ed in the pellet after treatment of the membrane with Triton X-100, it should be a membrane protein. Therefore, IF1 can bind not only to its inhibitory-binding site at the beta subunit of F1, but also to a non inhibitory site which is a membrane protein of approximate M(r) 5000-6 000.