Biochemical and Functional Analysis of Two Plasmodium falciparum Blood-Stage 6-Cys Proteins: P12 and P41

Abstract

The genomes of Plasmodium parasites that cause malaria in humans, other primates, birds, and rodents all encode multiple 6-cys proteins. Distinct 6-cys protein family members reside on the surface at each extracellular life cycle stage and those on the surface of liver infective and sexual stages have been shown to play important roles in hepatocyte growth and fertilization respectively. However, 6-cys proteins associated with the blood-stage forms of the parasite have no known function. Here we investigate the biochemical nature and function of two blood-stage 6-cys proteins in Plasmodium falciparum, the most pathogenic species to afflict humans. We show that native P12 and P41 form a stable heterodimer on the infective merozoite surface and are secreted following invasion, but could find no evidence that this complex mediates erythrocyte-receptor binding. That P12 and P41 do not appear to have a major role as adhesins to erythrocyte receptors was supported by the observation that antisera to these proteins did not substantially inhibit erythrocyte invasion. To investigate other functional roles for these proteins their genes were successfully disrupted in P. falciparum, however P12 and P41 knockout parasites grew at normal rates in vitro and displayed no other obvious phenotypic changes. It now appears likely that these blood-stage 6-cys proteins operate as a pair and play redundant roles either in erythrocyte invasion or in host-immune interactions.

Recruitment of Factor H as a Novel Complement Evasion Strategy for Blood-Stage Plasmodium falciparum Infection

“…4). Anti-Pf92 Abs did not detect Pf92 in the DPf92 line, whereas there was clear expression in both WT and DPf12 lines (33). WT and DPf92 merozoites were incubated in buffer, NHS, NHS-HI, or NHS-E to examine FH recruitment.…”

“…The 6-cys family is defined by domains containing six conserved cysteine residues and is unique to the Plasmodium species, which expresses multiple members of this family in different life stages (27)(28)(29). Merozoites express Pf12, Pf92, and Pf38 as GPI-anchored products, whereas Pf41 is peripherally associated (30)(31)(32)(33).…”

Structural and Biochemical Characterization of Plasmodium falciparum 12 (Pf12) Reveals a Unique Interdomain Organization and the Potential for an Antiparallel Arrangement with Pf41

“…Important Functional Role-Although a definitive functional role for Pf12 is lacking (21), studies have shown that Pf12 is one of 11 proteins that make up Ͼ94% of the GPI-anchored proteome of schizont stage parasites (Pf12 ϳ 5%) (20). To further investigate the functional consequences of Pf12, we performed bioinformatics and cellular localization studies.…”

“…Importantly, Pf12, Pf38 and Pf41 are strongly recognized by immune sera from naturally infected patients (17)(18)(19). Although Pf12, Pf38, and Pf92 are associated with the membrane through a GPI 4 anchor (20), it was recently established that soluble Pf41 (17) heterodimerizes with Pf12 to maintain membrane association (21), similar to the association of soluble Pfs230 with GPIanchored Pfs48/45 (7,22). In addition, negative results were reported for Pf12 in erythrocyte binding assays, and the Pf12 knock-out showed normal invasion (21).…”

“…Although Pf12, Pf38, and Pf92 are associated with the membrane through a GPI 4 anchor (20), it was recently established that soluble Pf41 (17) heterodimerizes with Pf12 to maintain membrane association (21), similar to the association of soluble Pfs230 with GPIanchored Pfs48/45 (7,22). In addition, negative results were reported for Pf12 in erythrocyte binding assays, and the Pf12 knock-out showed normal invasion (21). Ultimately, no well defined functional role has yet been ascribed to any of the four blood-stage 6-Cys proteins.…”