Evolution, Structure, and Function of Leucine rich Repeats as Revealed by Bioinformatics

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Leucine rich repeats (LRRs) are present in over 20,000 proteins from viruses to eukaryotes. Two to sixty-two LRRs occur in tandem. Most LRR proteins are involved in protein, ligand and in protein, protein interactions; these include the plant immune response and the mammalian innate immune response. Each repeat with 20-30 residues can be divided into a highly conserved segment and a variable segment. Eight classes have been recognized.Plant specific LRRs (class: PS-LRR) had previously been recognized in only plant proteins. However, we find that PS-LRRs are also present in twenty proteins from eleven bacterial species. We indicate that horizontal gene transfer of genes/gene fragments encoding PS-LRR domains occurred between bacteria and plants, as opposed to descent from a common ancestor. Also we find novel LRRs that were unrecognized until now. The novel LRR domains are present over three hundred proteins including fungal ECM33 protein from unicellular eukaryotes and bacteria.