The Echinococcus granulosus actin filament-fragmenting protein (EgAFFP) is a three domain member of the gelsolin family of proteins, which is antigenic to human hosts. These proteins, formed by three or six conserved domains, are involved in the dynamic rearrangements of the cytoskeleton, being responsible for severing and capping actin filaments and promoting nucleation of actin monomers. Various structures of six domain gelsolin-related proteins have been investigated, but little information on the structure of three domain members is available. In this work, the solution structure of the three domain EgAFFP has been investigated through small-angle x-ray scattering (SAXS) studies. EgAFFP exhibits an elongated molecular shape. The radius of gyration and the maximum dimension obtained by SAXS were, respectively, 2.52 ± 0.01 nm and 8.00 ± 1.00 nm, both in the absence and presence of Ca^sup 2+^. Two different molecular homology models were built for EgAFFP, but only one was validated through SAXS studies. The predicted structure for EgAFFP consists of three repeats of a central β-sheet sandwiched between one short and one long α-helix. Possible implications of the structure of EgAFFP upon actin binding are discussed.

The larval stage of the cestode tapeworm Echinococcus granulosus is the causative agent of cystic hydatid disease or hydatidosis, recognized as one of the world's major zoonoses (1). This parasite requires two mammalian hosts for completion of its life cycle. Adult tapeworms develop in the small intestine of definitive hosts (domestic dogs and wild canids), whereas the metacestode or hydatid cyst usually develops in the liver or lungs of intermediate hosts (mainly in ungulates, and accidentally in humans). The pathological effect of the disease is caused by the pressure exerted by the hydatid cyst on the intermediate host's viscera. Within the cyst, protoscoleces are produced by asexual reproduction and develop into the adult worm when ingested by the definitive host.

The E. granulosus complex life cycle involves important changes in cell morphology and physiology (2). The molecular and cellular mechanisms involved in E. granulosus development are still largely unknown but are likely to require extensive cytoskeleton reorganization (3,4).

The actin cytoskeleton is a vital component of several key cellular and developmental processes in eukaryotes, such as motility, cytokinesis, cytoplasmic organization, and endocytosis (5). In cells, the assembly and disassembly of actin filaments, in addition to their organization into functional three-dimensional (3D) networks, are regulated by a variety of actin-binding proteins (6-10). Among these proteins, those from the gelsolin superfamily control actin organization by severing filaments, capping filament ends, and nucleating actin assembly (11).

The best-studied members of this protein family are severin (12-14) and fragmin (15,16) from Dictyostelium discoideum and Physarum polycephalum, respectively, and gelsolin (17,18) and villin (19,20) from higher organisms. A common feature of this family is the segmentai organization into three (severin, fragmin) or six (gelsolin, villin) homologous domains that might have evolved from an ancestral one domain protein through a stepwise process, involving a gene triplication followed by an additional duplication event (21,17). The activities of these proteins are often modulated by signaling molecules, such as Ca^sup 2+^ or phosphorylated phosphoinositides (22). Based on gelsolin (23,24), the most extensively studied member of the family, it is generally accepted that the second domain binds F-actin, whereas the first domain (and the fourth one, for six domain proteins) binds G-actin.

So far, only a few proteins of the gelsolin superfamily have solved 3D structures. A search in the database of protein structures indicates that of all known members of this family to date, gelsolin is the only protein that has its fulllength (six domains) structure solved (25). Structures of other proteins, like villin (26) and severin (27,28), have been determined only for the first or second domains, usually bound to ligands (Ca^sup 2+^ and/or actin). Domain comparisons between the known structures show that they all share a common fold built around a central five-stranded mixed β-sheet, which is flanked by a long α-helix running parallel to the sheet and a short perpendicular running α-helix (25-29).

Our laboratory has previously cloned and functionally characterized a 42 kDa actin filament-fragmenting protein from E. granulosus (EgAFFP) (30). The recombinant EgAFFP protein is recognized by sera of ~69% of human hydatid disease patients (31) and, in vitro, was able to induce actin polymerization and sever actin filaments, confirming that it belongs to the gelsolin superfamily (30). According to sequence analysis, EgAFFP presents three repeated domains and is similar (36% identity) to the gelsolin NH^sub 2^-terminal half (G1-G3). The lack of structural data for full-length three domain members of the gelsolin superfamily, such as EgAFFP, represents an obstacle to the understanding of structure-function relationships of these smaller proteins, which are functionally equivalent to their six domain counterparts. Structural characterization of EgAFFP might help to understand how three domain members function and how they are regulated by calcium.