Identification of an endoplasmic reticulum-resident calcium-binding protein with multiple EF-hand motifs in asexual stages of Plasmodium falciparum.

An endoplasmic reticulum-located, calcium-binding protein, with an apparent molecular weight (Mr) of approximately 40,000 (PfERC), has been identified in the asexual stages of the malaria parasite, Plasmodium falciparum. This protein appears to be equivalent to a previously described gametocyte protein, Pfs40, which was reported to be expressed on the gametocyte surface (Rawlings DJ, Kaslow DC. J Biol Chem 1992;267:3976-3982). Sequencing of the 3' end of the gene revealed the omission of a single base in the 3' region of the published sequence. The corrected gene sequence encodes a C-terminal IDEL motif, which indicates residency of the 40 kDa protein within the endoplasmic reticulum. The predicted C-terminal region also appears to contain a sixth EF-hand calcium-binding domain, which suggests that PfERC is related to previously reported ER-localized calcium-binding proteins, namely reticulocalbin and ERC-55 (Ozawa M. J. Biochem. 1995;117:1113-1119; Weis K, Griffiths G, Lamond AI. J. Biol. Chem. 1994;269:19142-19150). The presence of the 40 kDa calcium-binding protein in malaria parasites was confirmed using 45Ca2+-blotting and partial protein sequencing of the corresponding Coomassie blue-stained polypeptide. Confocal immunofluorescence microscopy of asexual stage parasites was used to show that PfERC co-localizes with the known ER-located protein, Pfgrp. Analysis of immunoblots of tightly synchronized parasites showed that expression of PfERC increases with increasing maturity of the parasite. We propose that PfERC is a member of the reticulocalbin family of calcium-binding proteins and may play a role in protein trafficking in the malaria parasite.