CALSTORIN, A NEW CA2-PROTEIN OF THE MICROSOME LUMEN WHICH IS ABUNDANTIN THE RAT-BRAIN( BINDING)

A new Ca2+ binding protein (apparent Mr: 54 KDa; pI: 4.37; Stains All positive) which, based on N terminal sequence acid immunological crite ria, appears different from calsequestrin, calreticulin and the chaper onins, has been identified in the rat brain and recovered primarily in the microsome fraction. Carbonate extraction and trypsin digestion ex periments suggest the protein to be located within the microsome lumen . Its expression levels are considerable especially in the cerebellum (65% with respect to calreticulin). Ca-45 binding experiments on 2D bl ots suggest the protein to be of high capacity (higher than calreticul in) and low affinity (apparent Kd: 3 mM). These properties are typical of the proteins participating in the storage of Ca2+ within rapidly e xchanging organelles. The tentative name of calstorin (calcium-storage -protein, CST) is therefore proposed. (C) 1994 Academic Press, Inc.