Calreticulin: not just another calcium-binding protein.

In this paper we review some of the rapidly expanding information about calreticulin, a Ca(2+)-binding/storage protein of the endoplasmic reticulum. The emphasis is placed on the structure and function of calreticulin. We believe that calreticulin is a multifunctional Ca(2+)-binding protein and that distinct functional properties of the protein may be localized to each of the three structural domains of calreticulin. Most evidence indicates that calreticulin is a resident endoplasmic reticulum protein. However, it can also be found outside of the endoplasmic reticulum compartment, i.e. in the nuclear envelope, in the nucleus, in the cytotoxic granules in T-lymphocytes and in acrosomal vesicles of sperm cells. The evidence reviewed here clearly suggests that calreticulin has other functions in addition to its role as a Ca2+ storage protein in the endoplasmic reticulum.