Figure 1.

Ca²⁺ Binding Sites Are Poorly Conserved in VLN1.

A sequence alignment of plant villins based on the structural features determined for gelsolin. The six gelsolin domains are aligned against each other and labeled G1 to G6. These domains are ordered so that the most similar domains are paired (G1:G4, G2:G5, and G3:G6; Choe et al., 2002). GTAIL denotes the C-terminal extension, the tail latch. The boxed residues, labeled “Structurally Variable Region,” do not show general structural homology between the six domains of gelsolin. Horizontal bars above the sequence show secondary structure and are labeled by letter according to the convention in Burtnick et al. (1997). Orange and red bars signify α-helices in the active and inactive states, respectively; similarly, light and dark blue bars represent β-strands in the active and inactive states (Burtnick et al., 1997, 2004; Choe et al., 2002). Human villin (hV; accession no. NP_009058), the five Arabidopsis villins (VLN1–5; NP_029567, NP_565958, NP_567048, AAO64915, NP_200542), and 135-ABP (AAD54660) are aligned against the gelsolin domains. Structural homology is represented as follows: the gelsolin domain signature residues are highlighted in pink; residues that form the active state, domain-domain interactions between G2:G3, G5:G6, and G4:G6 are colored dark blue, light blue, and gray, respectively; tail-latch residues that interact with G2, in the inactive conformation, are shown in red type; residues that interact with actin are depicted with red highlighting; type 1 and type 2 calcium ion coordinating residues are shown with green and yellow highlighting, respectively. Red Xs denote residues implicated in Ca²⁺ regulation by mutagenesis of vertebrate villin (Kumar et al., 2004).