Comparative structural modeling of plant START domains. Ribbon diagrams of three-dimensional models comprised β-sheets (yellow), α-helices (red) and loops (green), showing three categories of cavity volume. The hydrophobic tunnels are represented as a white mesh. (A) Crystal structure of the mammalian START domain from STARD4. (B, C, D) Examples of small, medium and large cavity volumes, respectively, from modeled Arabidopsis START domains. (E) Superimposition of cavities from a mammalian domain (red mesh) and a model of an Arabidopsis domain (green mesh) to illustrate the amino acid residues that are predicted to change the dimensions of the cavity by blockage or expansion in several plant START domains. A Trp residue (green, marked `block’) in the plant structure corresponding to M224 (StAR numbering) results in shortening of the cavity length. In contrast, the mammalian cavity is restricted by W187 (red, marked `expand’), which corresponds to a smaller Val residue in the plant START domain, resulting in an increase in width at the center of the cavity.