Abstract
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 (N1) is greater than the number of the following block (N2); N1 » N2 in which N1 is variable in the homologs of individual families, while N2 is highly conserved. The five families of the LRR-RLKs except for the RPK2 family show N1 = 8 − 18 and N2 = 3 − 5. The nine families of the LRR-RLPs show N1 = 12 − 33 and N2 = 4; while N1 = 6 and N2 = 4 for the rice Os10g0479700 family and the N1 = 4 − 28 and N2 = 4 for the soybean protein family. The rule of N1 » N2 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.
Highlights
LRR regions are present in over 14,000 proteins in the data bases-PFAM, SMART, PROSITE, and InterPro [1,2,3,4]
What is the evolutionary origin of non-LRR island regions (IRs) interrupting LRRs? Previous research provided evidence that a direct duplication of the super motifs containing non-LRR regions naturally leads to the occurrence of non-LRR IRs in LRR@IR proteins, including LRR-containing 17 protein (LRRC17), LRRC32, LRR33, chondroadherin-like protein, trophoblast glycoprotein precursor, and Leishmania proteophosphoglycans, not from plants but from other eukaryotes [47]
The above observations indicate that the LRR-RLKs and LRR-RLPs evolved from gene duplications and recombination [39]
Summary
LRR (leucine rich repeat) regions are present in over 14,000 proteins in the data bases-PFAM, SMART, PROSITE, and InterPro [1,2,3,4]. Plant specific LRRs (class: PS-LRR) are 23 to 25 residues long and contain a conserved consensus sequence of the VS part, SGxIPxxLxxLxx, in which “S” is Ser or Thr,. Several classes contain LRR-containing receptor-like kinases (LRR-RLKs) [18,19], LRR-containing receptor-like proteins (LRR-RLPs) [20], nucleotide binding site LRR (NBS-LRR) proteins [21,22] and PGIPs [23,24,25] They provide an early warning system for the presence of potential pathogens and activate protective immune signaling in plants [26,27,28]. To identify non-LRR IRs, a method (called LRR@IRpred) utilizing LRRpred was developed and used to find LRR@IR proteins from organisms other than plants [47]. The structure, function and evolution of the LRR domains as well as the LRR@IR proteins are discussed
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