Article

Molecular Basis for Interaction of let-7 MicroRNAs with Lin28

Department of Biological Chemistry and Molecular Pharmacology, Harvard Medical School, Boston, MA 02115, USA.
Cell (Impact Factor: 33.12). 11/2011; 147(5):1080-91. DOI: 10.1016/j.cell.2011.10.020
Source: PubMed

ABSTRACT 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-7 g 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.

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    • "As a result of this knockdown, mature let-7 family miRNAs accumulated , as measured by RT-PCR, relative to transfection of a nontarget (siNT) control (Figure S3C). Also of note, siRNAs against both homologs were necessary to observe an induction of the let-7 family members, demonstrating their known semiredundant roles in let-7 maturation (Nam et al., 2011). "
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    • "While Lin28B represses let-7 processing in the nucleus to prevent the formation of the precursor form from the primary let-7, Lin28A also blocks cytoplasmic processing of let-7 [6]. It has recently been shown in mouse that deletion of the Lin28 linker domain alters the protein’s three-dimensional structure and is sufficient to disrupt sequestration of the precursor form of let-7 (pre-let-7) [14]. "
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    • "For example, Gln29 can recognize all four bases by a simple adjustment of its side-chain position and orientation. Alternative side-chain conformations have also been suggested to contribute to the degenerate recognition of polypyrimidine tracts by U2AF65 (45), the recognition of multiple RNA targets by the specific fem-3 binding factor (FBF) protein of the PUMILIO/FBF (PUF) family (46) or three different RNA targets by Lin28 (47). Very recently, the X-ray crystal structures of Pot1pC in complex with its cognate and several non-cognate ssDNA ligands have nicely demonstrated that changes in nucleic acid and protein structure (such as rotation of bases or protein side-chains) allows for alternative H-bond networks or stacking interactions and thus for the accommodation of several different DNA sequences by the same protein with thermodynamic equivalence (48). "
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