Super-motifs and evolution of tandem leucine-rich repeats within the small proteoglycans?biglycan, decorin, lumican, fibromodulin, PRELP, keratocan, osteoadherin, epiphycan, and osteoglycin

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Abstract
Leucine‐rich repeats (LRRs) with 20–30 amino acids in unit length are present in many proteins from prokaryotes to eukaryotes. The LRR‐containing proteins include a family of nine small proteoglycans, forming three distinct subfamilies: class I contains biglycan/PG‐I and decorin/PG‐II; class II: lumican, fibromodulin, PRELP, keratocan, and osteoadherin; and class III: epiphycan/PG‐Lb and osteoglycin or osteoinductive factor. Comparative sequence analysis of the 34 available protein sequences reveals that these proteoglycans have two types of LRRs, which we call S and T. The type S LRR is 21 residues long and has the consensus sequence of xxaPzxLPxxLxxLxLxxNxI. The type T LRR has 26 residues; its consensus sequence is zzxxaxxxxFxxaxxLxxLxLxxNxL. In both “x” indicates variable residue; “z” is frequently a gap; “a” is Val, Leu, or Ile; and I is Ile or Leu. These type S and T LRRs are ordered into two super‐motifs—STT with about 73 residues in classes I and II and ST with about 47 residues in class III. The 12 LRRs in the small proteoglycans of I and II are best represented as (STT)4; the seven LRRs of class III as (ST)T(ST)2. Our analyses indicate that classes I/II and III evolved along different paths after the establishment of the precursor ST, and classes I and II also diverged after the establishment of the precursor (STT)4. Proteins 2000;38:210–225.