RNA protein interactions governing expression of the most abundant protein in human body, type I collagen

Type I collagen is the most abundant protein in human body. The protein turns over slowly and its replacement synthesis is low. However, in wound healing or in pathological fibrosis the cells can increase production of type I collagen several hundred fold. This increase is predominantly due to posttranscriptional regulation, including increased half-life of collagen messenger RNAs (mRNAs) and their increased translatability. Type I collagen is composed of two alpha 1 and one alpha 2 polypeptides that fold into a triple helix. This stoichiometry is strictly regulated to prevent detrimental synthesis of alpha 1 homotrimers. Collagen polypeptides are co-translationally modified and the rate of modifications is in dynamic equilibrium with the rate of folding, suggesting coordinated translation of collagen alpha 1(I) and alpha 2(I) polypeptides. Collagen alpha 1(I) mRNA has in the 3' untranslated region (UTR) a C-rich sequence that binds protein alpha CP, this binding stabilizes the mRNA in collagen producing cells. In the 5' UTR both collagen mRNAs have a conserved stem-loop (5' SL) structure. The 5' SL is critical for high collagen expression, knock in mice with disruption of the 5' SL are resistant to liver fibrosis. the 5' SL binds protein LARP6 with strict sequence specificity and high affinity. LARP6 recruits RNA helicase A to facilitate translation initiation and associates collagen mRNAs with vimentin and nonmuscle myosin filaments. Binding to vimentin stabilizes collagen mRNAs, while nonmuscle myosin regulates coordinated translation of alpha 1(I) and alpha 2(I) mRNAs. When nonmuscle myosin filaments are disrupted the cells secrete only alpha 1 homotrimers. Thus, the mechanism governing high collagen expression involves two RNA binding proteins and development of cytoskeletal filaments. (C) 2013 John Wiley & Sons, Ltd.