Endoplasmic reticulum of rat liver contains two proteins closely related to skeletal sarcoplasmic reticulum Ca-ATPase and calsequestrin.

Rat liver endoplasmic reticulum (ER) membranes were investigated for the presence of proteins having structural relationships with sarcoplasmic reticulum (SR) proteins. Western immunoblots of ER proteins probed with polyclonal antibodies raised against the 100-kDa SR Ca-ATPase of rabbit skeletal muscle identified a single reactive protein of 100 kDa. Also, the antibody inhibited up to 50% the Ca-ATPase activity of isolated ER membranes. Antisera raised against the major intraluminal calcium binding protein of rabbit skeletal muscle SR, calsequestrin (CS), cross-reacted with an ER peptide of about 63 kDa, by the blotting technique. Stains-All treatment of slab gels showed that the cross-reactive peptide stained metachromatically blue, similarly to SR CS. Two-dimensional electrophoresis (Michalak, M., Campbell, K. P., and MacLennan, D. H. (1980) J. Biol. Chem. 255, 1317-1326) of ER proteins showed that the CS-like component of liver ER, similarly to skeletal CS, fell off the diagonal line, as expected from the characteristic pH dependence of the rate of mobility of mammalian CS. In addition, the CS-like component of liver ER was released from the vesicles by alkaline treatment and was found to be able to bind calcium, by a 45Ca overlay technique. From these findings, we conclude that a 100-kDa membrane protein of liver ER is the Ca-ATPase, and that the peripheral protein in the 63-kDa range is closely structurally and functionally related to skeletal CS.