Structure is an important and useful character to classify evolutionarily related proteins. For interleukin, IL1-like cytokines can be identified from other cytokines by a fold rich in beta-strands. IL6, IL11, IL12A, IL23A, IL27A, IL31 and oncostatin M (OSM) are long-chain class I helical cytokines. However, IL2, IL3, IL4, IL5, IL7, IL9, IL13, IL15 and IL21 are short-chain helical cytokines. The IL10-like and IL28-like are similar in structure through a fold that contains six or seven stacked α-helices. IL17 family members are structurally unrelated to other IL subfamily. IL17F shares a high degree of sequence homology with IL17A. IL17F was recently crystalized and structural analysis, finding a cysteine-knot fold.
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