Gliding motility in apicomplexan parasites.

Apicomplexan parasites, including Plasmodium and Toxoplasma, employ a unique form of substrate-dependent locomotion known as gliding motility. In these obligate, intracellular parasites, gliding motility is used for migration through the tissues and cells of the host, for active penetration of the host cell, and, at times, for proactive egress from the host. Gliding motility is powered by an actin-myosin based motor apparatus, known as the glideosome, which is situated within the elaborate cortical domain of the parasite. In this system, myosin is anchored to an internal membrane complex and drives the rearward translocation of actin-associated cell surface adhesins, thus leading to forward movement of the parasite. This review outlines our current understanding of glideosome architecture and the molecular basis of parasite motility.

Comparative ultrastructure and molecular phylogeny of Selenidium melongena n. sp. and S. terebellae Ray 1930 demonstrate niche partitioning in marine gregarine parasites (apicomplexa).

Gregarine apicomplexans are a diverse group of single-celled parasites that have feeding stages (trophozoites) and gamonts that generally inhabit the extracellular spaces of invertebrate hosts living in marine, freshwater, and terrestrial environments. Inferences about the evolutionary morphology of gregarine apicomplexans are being incrementally refined by molecular phylogenetic data, which suggest that several traits associated with the feeding cells of gregarines arose by convergent evolution. The study reported here supports these inferences by showing how molecular data reveals traits that are phylogenetically misleading within the context of comparative morphology alone. We examined the ultrastructure and molecular phylogenetic positions of two gregarine species isolated from the spaghetti worm Thelepus japonicus: Selenidium terebellaeRay 1930 and S. melongena n. sp. The ultrastructural traits of S. terebellae were very similar to other species of Selenidium sensu stricto, such as having vermiform trophozoites with an apical complex, few epicytic folds, and a dense array of microtubules underlying the trilayered pellicle. By contrast, S. melongena n. sp. lacked a comparably discrete assembly of subpellicular microtubules, instead employing a system of fibrils beneath the cell surface that supported a relatively dense array of helically arranged epicytic folds. Molecular phylogenetic analyses of small subunit rDNA sequences derived from single-cell PCR unexpectedly demonstrated that these two gregarines are close sister species. The ultrastructural differences between these two species were consistent with the fact that S. terebellae infects the inner lining of the host intestines, and S. melongena n. sp. primarily inhabits the coelom, infecting the outside wall of the host intestine. Altogether, these data demonstrate a compelling case of niche partitioning and associated morphological divergence in marine gregarine apicomplexans.

Myosin diversity in the diatom Phaeodactylum tricornutum.

This report describes the domain architecture of ten myosins cloned from the pennate diatom Phaeodactylum tricornutum. Several of the P. tricornutum myosins show similarity to myosins from the centric diatom Thalassiosira pseudonana as well as to one myosin from the oomycete Phytophthora ramorum. The P. tricornutum myosins, ranging in size from 126 kDa to over 250 kDa, all possess the canonical head, neck and tail domains common to most myosins, though variations in each of these domains is evident. Among the features distinguishing several of the diatom myosin head domains are N-terminal SH3-like domains, variations in or near the P-loop and Loop 1 regions close to the nucleotide binding pocket, and extended converter domains. Variations in the length of the neck domain or lever arm, defined by the light chain-binding IQ motifs, are apparent with the different diatom myosins predicted to contain from one to nine IQ motifs. Protein domains found within the P. tricornutum myosin tails include regions of coiled-coil structure, ankyrin repeats, CBS domain pairs, a PB1 domain, a kinase domain and a FYVE-finger motif. As many of these features have never before been characterized in myosins of any type, it is likely that these new diatom myosins will expand the repertoire of known myosin behaviors.

GpMyoF, a WD40 repeat-containing myosin associated with the myonemes of Gregarina polymorpha.

This study presents the first characterization of a WD40 repeat-containing myosin identified in the apicomplexan parasite Gregarina polymorpha. This 222.7 kDa myosin, GpMyoF, contains a canonical myosin motor domain, a neck domain with 6 IQ motifs, a tail domain containing short regions of predicted coiled-coil structure, and, most notably, multiple WD40 repeats at the C-terminus. In other proteins such repeats assemble into a beta-propeller structure implicated in mediating protein-protein interactions. Confocal microscopy suggests that GpMyoF is localized to the annular myonemes that gird the parasite cortex. Extraction studies indicate that this myosin shows an unusually tight association with the cytoskeletal fraction and can be solubilized only by treatment with high pH (11.5) or the anionic detergent sarkosyl. This novel myosin and its homologs, which have been identified in several related genera, appear to be unique to the Apicomplexa and represent the only myosins known to contain the WD40 domain. The function of this myosin in G. polymorpha or any of the other apicomplexan parasites remains uncertain.

Cellular and molecular mechanics of gliding locomotion in eukaryotes.

Gliding is a form of substrate-dependent cell locomotion exploited by a variety of disparate cell types. Cells may glide at rates well in excess of 1 microm/sec and do so without the gross distortion of cellular form typical of amoeboid crawling. In the absence of a discrete locomotory organelle, gliding depends upon an assemblage of molecules that links cytoplasmic motor proteins to the cell membrane and thence to the appropriate substrate. Gliding has been most thoroughly studied in the apicomplexan parasites, including Plasmodium and Toxoplasma, which employ a unique assortment of proteins dubbed the glideosome, at the heart of which is a class XIV myosin motor. Actin and myosin also drive the gliding locomotion of raphid diatoms (Bacillariophyceae) as well as the intriguing form of gliding displayed by the spindle-shaped cells of the primitive colonial protist Labyrinthula. Chlamydomonas and other flagellated protists are also able to abandon their more familiar swimming locomotion for gliding, during which time they recruit a motility apparatus independent of that driving flagellar beating.

Actin and myosin in Gregarina polymorpha.

Actin and two class XIV unconventional myosins have been cloned from Gregarina polymorpha, a large protozoan parasite inhabiting the gut of the mealworm Tenebrio molitor. These proteins were most similar to their homologues expressed in the coccidian and haemosporidian Apicomplexa such as Toxoplasma and Plasmodium despite the significant morphological differences among these parasites. Both actin and G. polymorpha myosin A (GpMyoA), a 92.6-kDa protein characterized by a canonical myosin head domain and short, highly basic tail, localized to both the longitudinally-disposed surface membrane folds (epicytic folds) of the parasite as well as to the subjacent rib-like myonemes that gird the parasite cortex. G. polymorpha myosin B (GpMyoB), a 96.3-kDa myosin, localized exclusively to the epicytic folds of the parasite. Both myosins were tightly associated with the cortical cytoskeleton and were solubilized only with a combination of high salt and detergent. Both GpMyoA and GpMyoB could bind to actin in an ATP-sensitive fashion. The distribution of actin and the unconventional myosins in G. polymorpha was consistent with their proposed participation in both the rapid (1-10 microm/sec) gliding motility exhibited by the gregarines as well as the myoneme-mediated bending motions that have been observed in these parasites.

Gliding motility: the molecules behind the motion.

In apicomplexan parasites, gliding motility and host cell invasion are driven by an actomyosin-based system. Recent studies have characterized several components of the gliding motility apparatus and have provided new insight into the molecular architecture of this locomotory system.