Plasmodium falciparum myosins: Transcription and translation during asexual parasite development

Abstract

Six myosins genes are now annotated in the Plasmodium falciparum Genome Project. Malaria myosins have been named alphabetically; accordingly, we refer to the two latest additions as Pfmyo-E and Pfmyo-F. Both new myosins contain regions characteristic of the functional motor domain of "true" myosins and, unusually for P. falciparum myosins, Pfmyo-F encodes two consensus IQ light chain-binding motifs. Phylogenetic analysis of the 17 currently known apicomplexan myosins together with one representative of each myosin class clusters all but one of the apicomplexan sequences together in Class XIV. This refines the earlier definition of the Class XIV Subclasses XIVa and XIVb. RT-PCR on blood stage parasite mRNA amplifies a specific product for all six myosins and each shows developmentally regulated transcription. Thus: Pfmyo-A and Pfmyo-B genes are transcribed throughout development; Pfmyo-C is predominant in trophozoites; Pfmyo-D occurs in trophozoites and schizonts; Pfmyo-E though barely present in earlier stages is abundant in schizonts; Pfmyo-F increases steadily throughout development and maturation. It is known that Pfmyo-A and Pfmyo-B are synthesised during late schizogony and we now show that Pfmyo-D expression is also temporally regulated to late trophozoites and schizonts where it distributes close to segregating nuclei. Thus, in asexual stages myosin synthesis does not always parallel transcript accumulation, showing that translation is also regulated. The implication is that the mRNAs are either subjected to turnover, synthesised and degraded, or that they are sequestered in an inactivate form until required for protein synthesis.

New insights into myosin evolution and classification

“…Class XIV had previously been described as exclusively comprising apicomplexan myosins and was divided into two subclasses (25,26). In contrast, our analyses (Fig.…”

“…2 and Table 2). That this robust association had not been recognized previously (4,19,25,26), with the ciliate myosins recently having been assigned to a separate class (19), is most likely because of the inclusion of rogue sequences, small dataset size, and the use of full-length myosin sequences in previous analyses. Based on our strong statistical support for this extended class XIV and on independent corroborating evidence (see below), we now include these 12 ciliate myosins in class XIV as subclass XIVd (Fig.…”

“…), often independently for each species (22)(23)(24)(25)(26)(27)(28). This has led to the confusing situation where nonorthologous apicomplexan sequences were designated with the same letter, e.g., PfMyoC (PfM-C) and TgMyoC of Plasmodium falciparum and T. gondii, respectively (25). We therefore propose a systematic naming convention for apicomplexan myosins, such that the same letters denote homologous myosins across taxa.…”

Myosin genes in Tetrahymena

“…In the apicomplexa, a group of parasitic protists that includes Babesia, Gregarina, Plasmodium, and Toxoplasma, 17 myosins have been identified; all but one cluster in class XIV, and all have been identified only in apicomplexa [Chaparro-Olaya et al, 2005;Heintzelman and Schwartzman, 1997;Heintzelman, 2004;Hettmann et al, 2000;Lew et al, 2002]. In the ciliophora, Whether the myosins in Tetrahymena are limited to this organism will have to await further identification of myosins in other protists.…”

“…Although manual inspection did not detect apparent IQ motifs, it is possible that degenerate IQ motifs exist in Tetrahymena myosins and were not detected by domain prediction tools. Other myosins that lack IQ motifs include most of class XIV myosins [Chaparro-Olaya et al, 2005] and class XVII myosins [Fujiwara et al, 1997].…”