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 app arent molecular weight (M-r) of approximate to 40000 (PfERC), has been identified in the asexual stages of the malaria parasite: Plasmodium falciparum. This protein appears to be equivalent to a previously desc ribed 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 corr ected gene sequence encodes a C-terminal IDEL motif, which indicates r esidency of the 40 kDa protein within the endoplasmic reticulum. The p redicted C-terminal region also appears to contain a sixth EF-hand cal cium-binding domain, which suggests that PfERC is related to previousl y reported ER-localized calcium-binding proteins, namely reticulocalbi n and ERC-55 (Ozawa M. J. Biochem. 1995;117:1113-1119; Weis K, Griffit hs 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 Ca-45(2+)-blotting and partial protein sequencing of the corres ponding Coomassie blue-stained polypeptide. Confocal immunofluorescenc e microscopy of asexual stage parasites was used to show that PfERC co -localizes with the known ER-located protein, Pfgrp. Analysis of immun oblots of tightly synchronized parasites showed that expression of PfE RC increases with increasing maturity of the parasite. We propose that PfERC is a member of the reticulocalbin family of calcium-binding pro teins and may play a role in protein trafficking in the malaria parasi te. (C) 1997 Elsevier Science B.V.