Expression and purification of mammalian calreticulin in Pichia pastoris

Calreticulin is a 46-kDa Ca2+-binding chaperone of the endoplasmic reticulum membranes. The protein binds Ca2+ with high capacity, affects intracellular Ca2+ homeostasis, and functions as a lectin-like chaperone. In this study, we describe expression and purification procedures for the isolation of recombinant rabbit calreticulin. The calreticulin was expressed in Pichia pastoris and purified to homogeneity by DEAE-Sepharose and Resource Q FPLC chromatography. The protein was not retained in the endoplasmic reticulum of Pichia pastoris but instead it was secreted into the external media. The purification procedures reported here for recombinant calreticulin yield homogeneous preparations of the protein by SDS-PAGE and mass spectroscopy analysis. Purified calreticulin was identified by its NH2-terminal amino acid sequences, by its Ca2+ binding, and by its reactivity with anti-calreticulin antibodies. The protein contained one disulfide bond between (88)Cys and (120)Cys. CD spectral analysis and Ca2+-binding properties of the recombinant protein indicated that it was correctly folded. (C) 2000 Academic Press.