Abstract
In inside-out human red cell membrane vesicles /IOV/, in the absence of Mg 2+, the only calcium-induced labelling by γ 32P-ATP occurs in a 140–150 000 molecular weight protein fraction, representing the hydroxylamine-sensitive phosphorylated intermediate /EP/ of the calcium pump. In the presence of Mg 2+ calcium-induced phosphorylation is accelerated but several other membrane proteins are also phosphorylated through protein kinase action forming hydroxylamine-insensitive bonds. Addition of calmodulin accelerates EP formation both in the absence and presence of Mg 2+. Treatment of the membrane with SH-group reagents significantly reduces EP formation. Mild trypsin digestion of IOVs, stimulating active calcium transport, eliminates calmodulin action and decreases the steady-state level of EP. In trypsin-digested IOVs the molecular weight of the 32P-labelled EP is shifted to lower values /110–120 000/ We suggest that trypsin digestion cleaves off a 20–40 000 molecular weight calmodulin-binding regulatory subunit of the calcium pump molecule.
Published Version
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