title: Malaria parasite actin filaments are very short.
abstract: A novel form of actomyosin regulation has recently been proposed in which the polymerisation of new actin filaments regulates apicomplexan parasite motility. Here, we identified actin I in the merozoites of Plasmodium falciparum by mass spectrometry. The only post-translational modification is acetylation of the N terminus (acetyl-Gly-Glu-actin), while methylation of histidine 73, a common modification for actin, is absent. Results obtained with anti-actin antibodies suggest that, in contrast to a previous report, there is no actin-ubiquitin conjugate in merozoites. About half of the extracted monomeric actin polymerised and actin filaments could be sedimented at 500,000g. In contrast, centrifugation at 100,000g, conditions commonly used to sediment filamentous actin, yielded very little F-actin. In a functional characterisation using an in vitro motility assay, actin filaments moved over myosin at a velocity indistinguishable from that of rabbit skeletal actin. Filament length, however, was too short to be resolved by conventional fluorescence microscopy. On electron micrographs an average filament length of approximately 100nm was determined. We also identified by mass spectrometry proteins co-purifying with filamentous actin, which are potential actin-binding proteins. Our results demonstrate differences in actin filament dynamics for an apicomplexan parasite, which could be due to specific properties of the actin and/or actin-regulatory proteins.
authors.affiliation: Division of Physical Biochemistry, National Institute for Medical Research, The Ridgeway, Mill Hill, London NW7 1AA, UK. stephan.schmitz@nimr.mrc.ac.uk
authors.forename: Stephan
authors.initials: S
authors.lastname: Schmitz
authors.forename: Munira
authors.initials: M
authors.lastname: Grainger
authors.forename: Steven
authors.initials: S
authors.lastname: Howell
authors.forename: Lesley J
authors.initials: LJ
authors.lastname: Calder
authors.forename: Martina
authors.initials: M
authors.lastname: Gaeb
authors.forename: Jennifer C
authors.initials: JC
authors.lastname: Pinder
authors.forename: Anthony A
authors.initials: AA
authors.lastname: Holder
authors.forename: Claudia
authors.initials: C
authors.lastname: Veigel
mesh_terms: D008841:Actin Cytoskeleton / Q000737:chemistry* / Q000648:ultrastructure; D000199:Actins / Q000737:chemistry* / Q000302:isolation & purification; D000818:Animals; D013058:Mass Spectrometry; D008854:Microscopy, Electron; D010963:Plasmodium falciparum / Q000378:metabolism* / Q000648:ultrastructure; D011499:Protein Processing, Post-Translational; D020539:Sequence Analysis, Protein
chemical_list: D000199:Actins