MicroRNAs (miRNAs) are small noncoding RNA molecules that regulate gene expression. Among these, members of the let-7 miRNA family control many cell-fate determination genes to influence pluripotency, differentiation, and transformation. Lin28 is a specific, posttranscriptional inhibitor of let-7 biogenesis. We report crystal structures of mouse Lin28 in complex with sequences from let-7d, let-7-f1, and let-7g precursors. The two folded domains of Lin28 recognize two distinct regions of the RNA and are sufficient for inhibition of let-7 in vivo. We also show by NMR spectroscopy that the linker connecting the two folded domains is flexible, accommodating Lin28 binding to diverse let-7 family members. Protein-RNA complex formation imposes specific conformations on both components that could affect downstream recognition by other processing factors. Our data provide a molecular explanation for Lin28 specificity and a model for how it regulates let-7.
Graphical Abstract
Highlights
► Crystal structures of Lin28:let-7 complexes reveal bipartite recognition ► Cold shock domain binds a closed loop and zinc knuckles bind the GGAG motif ► A flexible linker facilitates recognition of diverse let-7 sequences ► Lin28:let-7 interface mutations inhibit Lin28 activity in vitro and in vivo