Molecular modelling of RNases from almond involved in self-incompatibility

Abstract

Gametophytic self-incompatibility (GSI) is a natural mechanism in flowering plants, including almond and other fruit tree species, to prevent inbreeding and promote outcrossing. It is typically under the control of a specific locus, known as the S-locus, which contains at least two genes. The first gene encodes glycoproteins with ribonuclease (S-RNase) activity in the pistils, and the second is a specific F-box gene (SFB) expressed in the pollen. In Solanaceae, Scrophulariaceae and Rosaceae, active S-RNases in the style are essential for rejection of haploid pollen, when the S-allele of pollen matches one of two S-alleles of the diploid pistil. The S-RNase was first identified in Prunus more than 20 years ago, whereas SFB was identified only recently. In spite of the knowledge of the genetic structure of the female and male determinants of GSI, the nature of their mutual interactions at genetic and biochemical levels remain unclear. Thus, detailed understanding of the protein structure involved in GSI may help in discovering how proteins involved in GSI function and fulfil their biological roles. To this aim, three-dimensional (3D) models of a self-compatible (Sf) and a self-incompatible (S8) S-RNase of almond have been constructed, using comparative modelling tools. The molecular models of Sf and S8 showed that 3D architectures of their folds had the same topology as typical members of the RNase T2 family. The modelled structures consisted of mixed α and β folds, with six helices and six beta-strands.