Reprinted from Trends in Biochemical Sciences

The sarcoplasmic reticulum is a mem-brane system unique to muscle tissue. In skeletal muscle, where it surrounds each myofibril, the membrane can occupy a very large percentage of the total cell volume. It plays an important role in muscle contraction by regulating intra-celular Ca2+ concentrations. When the membrane system i s f r agmented i t reforms vesicles with the same orientation in vitro as in vivo. Because these vesicles are so specialized for Ca2+ transport and because they are available in large quantities they have provided an excellent system for investigation into the mechan-ism of Ca2+ transport [1,2]. Investigations in this area have been advanced by the isolation and characterization of the proteins of the membrane, and these advances have permitted the development of other areas of investigation such as the biosynthesis of the membrane. Protein composition The sarcoplasmic reticulum contains only a few proteins. The ATPase, mol. wt 100,000, constitutes about two-thirds of the total protein of the membrane but perhaps as much as 90 % of the intrinsic membrane protein [1]. The enzyme can be isolated in its active form containing phospholipid and small amounts of low molecular weight proteolipid which has a hydrophilic amino acid composition but contains covalently bound fatty acid. This enzyme, when incorporated into lipo-somes containing internal oxalate or phosphate, can catalyse ATP-dependent Ca2+ transport. Phospholipid is a co-facto