On subunit structure of Ca2+-dependent ATPase of sarcoplasmic reticulum.

The protein composition of sarcoplasmic reticulum membranes has been extensively characterized by means of polyacrylamide gel electrophoresis in one [l] or two [2] dimensions. It is generally accepted that the protein of mol. wt 100 000 (i.e., Ca’+ATPase) is the principal protein of sarcoplasmic reticulum. However, monomer and dimer fractions of deoxycholate-solubilized Ca”-ATPase during isoelectric focusing consistently gave rise to >5 different bands from pI 5.0-5.7 [3]. Later the Ca”-ATPase of rabbit sarcoplasmic reticulum has been shown [4,5] to consist of several different polypeptides of similar molecular weights whose isoelectric points were ~1-5-6. Using the techniques in [4] >3 major protein bands in normal and dystrophic chicken sarcoplasmic reticulum were observed [6]. These three bands were localized between pH 5.0-7.0. As far as the Ca”-ATPase of sarcoplasmic reticulum may represent an oligomeric complex of several subunits [7] it was interesting to investigate whether the enzyme is composed of different or identical polypeptides. The results reported here show that the Cap-ATPase of rabbit sarcoplasmic reticulum is composed of one type of polypeptide that has an apparent isoelectric point of p1 5.91 f 0.09 and mol. wt -100 000.