ACTIN-BINDING PROTEINS OF INVASIVE MALARIA PARASITES AND THE REGULATION OF ACTIN POLYMERIZATION BY A COMPLEX OF 32 34-KDA PROTEINS ASSOCIATED WITH HEAT-SHOCK-PROTEIN 70KDA/

Movement of the malaria parasite into a host erythrocyte during invasi on is thought to involve polymerization of parasite actin. We have use d F-actin affinity chromatography to isolate actin-binding proteins fr om Plasmodium knowlesi merozoites, in an attempt to identify proteins responsible for regulating parasite actin polymerization during invasi on. Five major proteins, of molecular masses 75, 70, 48, 40 and 34 kDa , were reproducibly eluted from the F-actin columns. The 70-kDa actin- binding protein was identified by tryptic peptide microsequencing as h eat shock protein-70 kDa (HSC70); this identification was confirmed by Western blotting with anti-HSC70 antibody, and binding of the protein to ATP-agarose. A doublet of 32/34-kDa proteins coeluted with parasit e HSC70 from the F-actin and ATP-agarose columns; a complex of these t hree proteins was also observed by gel filtration chromatography. High ly enriched fractions containing the Plasmodium HSC70/32/34 complex in hibited the polymerization of rabbit skeletal muscle actin, in vitro. This capping activity was calcium-independent, and abrogated by phosph atidylinositol 4,5-bisphosphate. The average length of the actin filam ents polymerized in presence of the HSC70/32/34-kDa complex was signif icantly shorter than in the absence of the complex, consistent with a capping activity. The capping or uncapping of actin filament ends by t he HSC70/32/34-kDa complex during invasion could provide a mechanism f or localized actin filament growth and movement of the parasite into t he host cell. (C) 1998 Elsevier Science B.V. All rights reserved.