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

AbstractActin 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 parasitePlasmodium berghei. In addition, we compare homo- and heterodimeric arrangements ofP. bergheiCPs (PbCPs) in solution. We complement our findings by describing the regulatory effects ofPbCPs on heterologous skeletal muscle α-actin as well as parasite actin. Comprehensive kinetic and steadystate measurements show atypical regulation of actin dynamics;PbCPs 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 heterodimericPbCPs show redundant effectsin 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 asymmetricPbCP 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 inPlasmodiumand the eligibility ofPbCPs as drug targets against malaria.Significance statementMalaria parasites of the genusPlasmodiumdisplay 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 uniquePlasmodium bergheiCP (PbCP) 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 ofPbCP and extensive biochemical data, describing the atypical behavior of eachPbCP form. ThePbCP 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.