Structure of an atypical homodimeric actin capping protein from the malaria parasite

Actin capping proteins (CPs) are essential regulators of actin dynamics in all eukaryotes. Their structure and function have been extensively characterized in higher eukaryotes but their role and mechanism of action in apicomplexan parasites remain enigmatic. Here, we present a crystal structure of a unique homodimeric CP from the rodent malaria parasite Plasmodium berghei . In addition, we compare homo- and heterodimeric arrangements of P. berghei CPs ( Pb CPs) in solution. We complement our findings by describing the regulatory effects of Pb CPs on heterologous skeletal muscle α-actin as well as parasite actin. Comprehensive kinetic and steadystate measurements show atypical regulation of actin dynamics; Pb CPs facilitate rapid turnover of parasite actin I without affecting the apparent critical concentration. Possibly to rescue actin filament capping in life cycle stages where the CP β-subunit is downregulated, homo- and heterodimeric Pb CPs show redundant effects in vitro . However, our data suggest that homodimers may in addition influence actin kinetics by recruiting lateral actin dimers. This unusual function could arise from the absence of a β-subunit, as the asymmetric Pb CP homodimer lacks the structural elements essential for canonical barbed end interactions, suggesting a novel CP binding mode. These findings facilitate further studies aimed at elucidating the precise actin filament capping mechanism in Plasmodium and the eligibility of Pb CPs as drug targets against malaria. Significance statement Malaria parasites of the genus Plasmodium display a unique form of gliding motility, which depends on an unconventional actomyosin motor. Actin capping proteins (CPs) play a major role in regulating parasite motility. Here, we describe a unique Plasmodium berghei CP ( Pb CP) system, behaving contradictory to canonical heterodimeric CPs, more suited to regulate the fast dynamics of the parasite actin system. We present the crystal structure of a distinctive homodimeric form of Pb CP and extensive biochemical data, describing the atypical behavior of each Pb CP form. The Pb CP homodimer displays capping even in the absence of canonical conserved structural elements, suggesting a novel actin-CP interaction mode. These distinct structural properties could provide opportunities for drug design against malaria.