Characterization of heart cytosolic proteins capable of modulating calcium uptake by the sarcoplasmic reticulum. 1. Isolation of a protein with protective activity and its identification as muscle albumin.

A new type of regulation of the Ca-pumping activity of isolated sarcoplasmic reticulum membranes has been investigated. An inhibitory and an antagonistic fraction were obtained after (NH4)2SO4 fractionation of cardiac muscle cytosol according to a published procedure [Narayanan et al. (1983) Biochem. Biophys. Acta 735, 53-66]. The former fraction inhibited Ca uptake by sarcoplasmic reticulum vesicles in a concentration-dependent way. The inhibition could be prevented and even reversed by addition of the antagonistic fraction. The protein components of this latter fraction were resolved and separated using an anion-exchange chromatographic procedure (mono Q column in an FPLC system). A pure protein component with antagonistic properties was isolated. Biochemical (molecular mass, tryptic digestion pattern and antagonistic activity) and immunological (cross-reactivity with specific antibodies) analysis resulted in the identification of the purified antagonist protein as muscle albumin, a serum-albumin-like protein which is localized near the A/I junctions in the striated muscle cells. The protein may be involved in the regulation of Ca fluxes across the cisternal compartments of the sarcoplasmic reticulum.