Leucine-Rich Repeat (LRR) Domains Containing Intervening Motifs in Plants

LRRs (leucine rich repeats) are present in over 14,000 proteins. Non-LRR, island regions (IRs) interrupting LRRs are widely distributed. The present article reviews 19 families of LRR proteins having non-LRR IRs (LRR@IR proteins) from various plant species. The LRR@IR proteins are LRR-containing receptor-like kinases (LRR-RLKs), LRR-containing receptor-like proteins (LRR-RLPs), TONSOKU/BRUSHY1, and MJK13.7; the LRR-RLKs are homologs of TMK1/Rhg4, BRI1, PSKR, PSYR1, Arabidopsis At1g74360, and RPK2, while the LRR-RLPs are those of Cf-9/Cf-4, Cf-2/Cf-5, Ve, HcrVf, RPP27, EIX1, clavata 2, fascinated ear2, RLP2, rice Os10g0479700, and putative soybean disease resistance protein. The LRRs are intersected by single, non-LRR IRs; only the RPK2 homologs have two IRs. In most of the LRR-RLKs and LRR-RLPs, the number of repeat units in the preceding LRR block (N(1)) is greater than the number of the following block (N(2)); N(1) » N(2 ) in which N(1) is variable in the homologs of individual families, while N(2) is highly conserved. The five families of the LRR-RLKs except for the RPK2 family show N(1) = 8 − 18 and N(2) = 3 − 5. The nine families of the LRR-RLPs show N(1) = 12 − 33 and N(2) = 4; while N(1) = 6 and N(2) = 4 for the rice Os10g0479700 family and the N(1) = 4 − 28 and N(2) = 4 for the soybean protein family. The rule of N(1) » N(2) might play a common, significant role in ligand interaction, dimerization, and/or signal transduction of the LRR-RLKs and the LRR-RLPs. The structure and evolution of the LRR domains with non-LRR IRs and their proteins are also discussed.