Structural basis of pre-let-7 miRNA recognition by the zinc knuckles of pluripotency factor Lin28

Lin28 prevents the processing of pre-let-7 RNAs, but it is not clear where the Lin28 RNA binding domains interact with the RNA. The NMR structure of the Lin28 zinc knuckles with a short RNA motif reveals that each knuckle interacts with an AG dinucleotide, allowing the determination of a consensus motif for pre-let-7 recognition. Lin28 inhibits the biogenesis of let-7 miRNAs through a direct interaction with the terminal loop of pre-let-7. This interaction requires the zinc-knuckle domains of Lin28. We show that the zinc knuckle domains of Lin28 are sufficient to provide binding selectivity for pre-let-7 miRNAs and present the NMR structure of human Lin28 zinc knuckles bound to the short sequence 5′-AGGAGAU-3′. The structure reveals that each zinc knuckle recognizes an AG dinucleotide separated by a single nucleotide spacer. This defines a new 5′-NGNNG-3′ consensus motif that explains how Lin28 selectively recognizes pre-let-7 family members. Binding assays in cell lysates and functional assays in cultured cells demonstrate that the interactions observed in the solution structure also occur between the full-length protein and members of the pre-let-7 family. The consensus sequence explains several seemingly disparate previously published observations on the binding properties of Lin28.