CROSS-LINKING OF THE (CA2+ + MG2+)-ATPASE PROTEIN

The addition of cupric-1,10,-phenanthroline, a cross-linking catalyst, to sarcoplasmic reticulum membranes caused protein sulfhydryl groups to form disulfide bridges. Following a short exposure to the catalyst (15 s, 22°C) most of the protein was in a dimeric form (Mr = 248 000). Longer exposure times resulted in the formation of trimers, tetramers and other oligomers too large to enter the gel. At low temperatures (4°C) dimer formation predominates, even for exposure times as long as 5 min. Cross-linking in the presence of 7.5 mM Triton X-100 (a concentration that resulted in clearing of the membrane suspension and thus solubilization of the membrane components) showed the appearance of a considerable dimer fraction, however, most of the (Ca2+ + Mg2+)-ATPase protein appeared as a monomer. Following 1 min of cross-linking at 22°C, freeze-etched membranes showed no alteration in the number or appearance of 80 Å intramembranous particles. Thus extensive cross-linking of the (Ca2+ + Mg2+)-ATPase protein can occur without disruption of the normal position of the intramembrane portion of the molecule. © 1979.