Geometric constrains for detecting short actin filaments by cryogenic electron tomography

PACS Codes: 87.64.EeActin filaments are important in many cellular processes including cell motility, cell division and organelle movement. Actin filaments (F-actin) are polymerized from monomers (G-actin) and their formation, disassembly and length is controlled by a large number of actin binding proteins [1]. Different arrays of actin filaments can be found in filopodia and lamellipodia, which are different structures important for cell motility [2]. Pathogens have found many ways of interfering with host cell actin dynamics to use this cytoskeletal component for their own end during invasion of or egress from a host cell [3,4]. In contrast to bacteria and viruses, parasites of the protist phylum of Apicomplexa, which are phylogenetically older than the common ancestor of animals and plants, can use their own actin cytoskeleton for motility through tissues and invasion of host cells [5,6]. Members of this phylum include malaria parasites (Plasmodia) and parasites causing Toxoplasmosis (Toxoplasma). In the invasive forms of these parasites the actin filaments are thought to be located in a narrow volume called the supra-alveolar space between the plasma membrane and an underlying alveolate-specific double membrane termed the alveoli or inner membrane complex (IMC) (Figure 1). The IMC is in turn connected to microtubules on its cytoplasmic face [7] (Figure 1). Actin is thought to be polymerized by a protein containing a formin homology domain [8]. Biochemical studies have shown that these actin filaments are very short, being only able to assemble in vitro into filaments of less than 150 nm in length [9-11]. Once polymerized these short filaments are thought to be linked via the glycolytic enzyme aldolase to a trans-membrane protein of the TRAP family, which spans the plasma membrane and thus links the parasite to its substrate [12-15]. An apicomplexa-specific myosin XIV has been shown to be anchored at the IMC and to provide the force for translocation [5,16,17]. E