Toxoplasma and Plasmodium protein kinases: Roles in invasion and host cell remodelling

Abstract

Some apicomplexan parasites have evolved distinct protein kinase families to modulate host cell structure and function. Toxoplasma gondii rhoptry protein kinases and pseudokinases are involved in virulence and modulation of host cell signalling. The proteome of Plasmodium falciparum contains a family of putative kinases called FIKKs, some of which are exported to the host red blood cell and might play a role in erythrocyte remodelling. In this review we will discuss kinases known to be critical for host cell invasion, intracellular growth and egress, focusing on (i) calcium-dependent protein kinases and (ii) the secreted kinases that are unique to Toxoplasma (rhoptry protein kinases and pseudokinases) and Plasmodium (FIKKs).

Introduction

Protein kinases mediate the transfer of phosphate groups from ATP to specific residues on their target proteins, resulting in changes in the activity, stability, interactions with ligands or localisation of the phosphorylated substrates. Kinases themselves are often similarly regulated, and many of them function as signalling mediators that integrate upstream signals in the form of second messengers, post translational modifications or binding of regulatory proteins. It is therefore not surprising that many parasites have evolved distinct protein kinase families with novel domain structures and biochemical features to regulate key parasite-specific physiological processes that must be executed in a timely fashion during development or in response to external stimuli and physiological cues from the host. As signalling mediators, protein kinases are also well suited to function at the host–parasite interface, where they can perturb host signalling pathways, activate dormant mechanisms in the host cell, or disrupt the proper functioning of host proteins.

Both Toxoplasma and Plasmodium contain a family of calcium-dependent protein kinases (CDPKs), whose occurrence is restricted to plants and Alveolates (the superphylum that comprises the ciliates and apicomplexans), although trypanosomatids also possess kinases with an EF-hand calcium binding domain thought to be phylogenetically distinct from plant and Alveolate CDPKs (Parsons et al., 2005). CDPKs have a domain structure consisting of a calcium-binding domain fused to the kinase domain, such that kinase activity is stimulated upon calcium binding. Studies of apicomplexan CDPKs have revealed a conserved regulatory mechanism and highlighted the importance of calcium in regulating a number of key physiological processes including host cell attachment and invasion, gliding motility and parasite egress.

Toxoplasma gondii, a highly prevalent obligate intracellular protozoan parasite, seems to rely for a large part on protein kinases and pseudo kinases to modify the host cell. Toxoplasma is capable of infecting all nucleated cells of most warm-blooded animals. This ability to establish a chronic infection in such a wide range of host species and cell types is likely associated with its ability to modify many aspects of the host cell’s normal physiology. It does this by secreting proteins from the rhoptry, a specialised secretory organelle that is found only in apicomplexan parasites, directly into the host cytosol where they can exert their function. Many rhoptry proteins have homology to kinases (Bradley et al., 2005) and a large proportion of them are predicted to be pseudokinases (Peixoto et al., 2010).

Malaria parasites (genus Plasmodium) also belong to the Apicomplexa, however, unlike Toxoplasma, they have a very restricted range of host cells which they can invade and in which they can replicate. After inoculation into the vertebrate host through the bite of an infected mosquito, Plasmodium sporozoites must first invade hepatocytes, where they undergo asexual proliferation (exo-erythrocytic schizogony), generating several thousand merozoites in the process. This strict dependence on hepatocytes for the pre-erythrocytic stage of infection has, however, recently been challenged by in vivo observations of Plasmodium berghei parasites developing in dermal and epidermal cells at the site of inoculation and generating infective merozoites (Gueirard et al., 2010).

The only cell type permissive for invasion by Plasmodium merozoites is the erythrocyte (or the reticulocyte in some instances). Invasion involves release of the contents of rhoptries and micronemes (another type of specialised secretory organelle) and formation of a parasitophorous vacuole (PV). The parasite causes considerable modifications to its host red blood cell (RBC) that include the establishment of a complex trafficking system that mediates translocation of parasite-encoded proteins to the RBC membrane skeleton and surface (Cooke et al., 2004). In the case of Plasmodium falciparum, the most lethal of the five species of malaria parasites that can infect humans, such proteins include components of the so-called “knob” structures, through which parasite-infected RBCs adhere to vascular endothelial cells, thereby significantly contributing to pathogenesis (Cooke et al., 2001). There are no orthologs of the Toxoplasma rhoptry kinases in malaria parasites, however, the P. falciparum kinome includes a family (20 members) of related kinase-like sequences called FIKKs, due to the presence of a Phe-Ile-Lys-Lys motif they share in their kinase domain (Ward et al., 2004). Most notably, 18 of the 20 fikk genes in P. falciparum are predicted to encode fully functional kinases, 16 of which are predicted to be exported into the host RBC (Schneider and Mercereau-Puijalon, 2005), and at least some of these enzymes have been shown to be exported to the host RBC and to be associated with kinase activity (Nunes et al., 2007, Nunes et al., 2010).

Many of the Toxoplasma and Plasmodium kinases are related to kinases with known functions in other eukaryotes and these will not be discussed in this review. Instead, we will discuss in more detail, kinases from these parasites that are known to be critical for host cell invasion, intracellular growth and parasite egress, focusing on (i) calcium-dependent protein kinases and (ii) the secreted kinases that are unique to Toxoplasma (ROPKs) and Plasmodium (FIKKs).