The Plasmodium circumsporozoite protein

Abstract

The circumsporozoite protein (CSP) is one of the most studied proteins of the malaria parasite. It is the target of the only licensed malaria vaccines and is essential for sporozoite formation and infectivity. Yet, the mechanisms by which CSP functions and its interactions with other proteins are only beginning to be understood. Here we review the current state of knowledge of CSP structure and function, as sporozoites develop in the mosquito and establish infection in the mammalian host, and outline outstanding questions that need to be addressed.

Keywords: GPI-anchor; adhesion; malaria; mosquito; oocyst.

Figure 1.. The circumsporozoite protein (CSP) defines the surface of sporozoites.

(A) The journey of Plasmodium sporozoites in the Anopheles mosquito and mammalian host. Sporozoites develop in oocysts on the mosquito midgut and when mature, exit into the hemolymph, and are transported to the salivary gland, which they actively enter. Salivary gland sporozoites are inoculated into the skin of the vertebrate host where they actively move to find and invade a blood vessel. The blood circulation takes them to the liver where they invade hepatocytes. Sporozoites are colored yellow or green depending on the conformation of CSP, with yellow indicating CSP in the αTSR exposed conformation, first observed in mid-stage oocysts, and green indicating CSP in the αTSR-masked conformation. Below is shown the processing states of CSP on the sporozoite plasma membrane during development in the oocyst, migration through the mosquito hemocoel and mammalian skin, activation in the liver sinusoid, and invasion into hepatocytes. The αTSR domain is represented in yellow, the N terminus is represented in green, and the red symbol indicates the yet-to-be-identified parasite protease that cleaves CSP. (B) Domain architecture of CSP. The N terminus following the signal peptide (SP) contains two Plasmodium export element (PEXEL) motifs (P1 and P2). The recognized sequence and cleavage sites are indicated below, also for Region I (RI). The central repeat region is then followed by the αTSR, an adhesion domain, and the GPI-anchor addition sequence. Below are alphafold models of the Plasmodium falciparum CSP αTSR-domain with a fucose group (blue pentose icon). The α-helix is indicated in red, the flexible flap in magenta, the locations of which are also shaded in the primary structure above. The hydrophobic pocket is marked with an asterisk. (C) Western blots of CSP using the indicated antibodies that recognize the domains as shown in (B), revealing processing of Region I. The N terminus after cleavage is not observed in western blots. Abbreviations: GPI, glycosylphosphatidylinositol; TSR, type I thrombospondin repeat.

Figure 2.. The circumsporozoite protein (CSP) is essential for sporozoite formation in oocysts.

(A) Oocyst development of wild-type Plasmodium parasites is shown. CSP starts to be expressed at the end of the oocyst growth phase, accumulating at the plasma membrane. Then the plasma membrane starts to invaginate, resulting in sporoblast formation with many small nuclei directly underlying the plasma membrane. At the surface of sporoblasts, sporozoite apical tips are formed, resulting in locally curved plasma membrane with underlying inner-membrane complex and subpellicular microtubules (both together depicted in red) as well as the rhoptry anlagen. The buds elongate into sporozoites, packaging the nuclei as well as mitochondria and apicoplast inside the forming sporozoites. (B) In the absence of CSP, the growth phase is directly followed by premature inner-membrane deposition and polymerization of subpellicular microtubules, which quickly underlay the entire plasma membrane. Some plasma membrane invagination does occur, resulting in failed sporozoite formation and the generation of multiple stacks of plasma membrane and inner-membrane complex in close proximity. The oocyst finally degrades and dies. Abbreviation: WT, wild type.