Characterization of sequence specific binding of LARP6 to the 5' stem-loop of type I collagen mRNAs and implications for rational design of antifibrotic drugs

Excessive synthesis of type I collagen characterizes fibrotic diseases. Binding of LARP6 to the 5 stem-loop (5SL) of collagen mRNAs regulates their translation and the high rate of biosynthesis in fibrosis. LARP6 needs two domains to form stable complex with 5SL RNA, the La-domain and the juxtaposed RRM domain (jointly called the La-module). We describe that the La-domain of LARP6 is necessary and sufficient for recognition of 5SL in sequence specific manner. The three amino acid motif, RNK, located in the flexible loop which connects the second -helix to the {beta}-sheet of the La domain is critical for binding. Mutation of any of these three amino acids abolishes the binding of La-domain to 5SL. The major site of crosslinking of LARP6 to 5SL RNA was mapped to this motif. The RNK motif is not found in other LARPs, which can not bind 5SL. Presence of RRM increases the stability of complex between La-domain and 5SL RNA and RRM domain does not make extensive contacts with 5SL RNA. We propose a model in which the initial recognition of 5SL by LARP6 is mediated by the RNK epitope and further stabilized by the RRM domain. This discovery suggests that the interaction between LARP6 and collagen mRNAs can be blocked by small molecules that target the RNK epitope and will help rational design of the LARP6 binding inhibitors as specific antifibrotic drugs.