Leucine-rich repeat
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Through Protein, Amino acid and Turn (biochemistry)
Leucine-rich repeats are frequently involved in the formation of protein–protein interactions.
Examples
Leucine-rich repeat motifs have been identified in a large number of functionally unrelated proteins. The best-known example is the ribonuclease inhibitor, but other proteins such as the tropomyosin regulator tropomodulin and the toll-like receptor also share the motif. In fact, the toll-like receptor possesses 10 successive LRR motifs which serve to bind pathogen- and danger-associated molecular patterns.
Although the canonical LRR protein contains approximately one helix for every beta strand, variants that form beta-alpha superhelix folds sometimes have long loops rather than helices linking successive beta strands.
One leucine-rich repeat variant domain (LRV) has a novel repetitive structural motif consisting of alternating alpha- and 310-helices arranged in a right-handed superhelix, with the absence of the beta-sheets present in other leucine-rich repeats.
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Associated domains
Leucine-rich repeats are often flanked by N-terminal and C-terminal cysteine-rich domains, but not always as is the case with C5orf36
They also co-occur with LRR adjacent domains. These are small, all beta strand domains, which have been structurally described for the protein Internalin (InlA) and related proteins InlB, InlE, InlH from the pathogenic bacterium Listeria monocytogenes. Their function appears to be mainly structural: They are fused to the C-terminal end of leucine-rich repeats, significantly stabilising the LRR, and forming a common rigid entity with the LRR. They are themselves not involved in protein-protein-interactions but help to present the adjacent LRR-domain for this purpose. These domains belong to the family of Ig-like domains in that they consist of two sandwiched beta sheets that follow the classical connectivity of Ig-domains. The beta strands in one of the sheets is, however, much smaller than in most standard Ig-like domains, making it somewhat of an outlier.
An iron sulphur cluster is found at the N-terminus of some proteins containing the leucine-rich repeat variant domain (LRV). These proteins have a two-domain structure, composed of a small N-terminal domain containing a cluster of four Cysteine residues that houses the 4Fe:4S cluster, and a larger C-terminal domain containing the LRV repeats. Biochemical studies revealed that the 4Fe:4S cluster is sensitive to oxygen, but does not appear to have reversible redox activity.