MicroRNAs (miRNAs) regulate the expression of a large number of protein-coding genes. Their primary transcripts (pri-miRNAs) have to undergo multiple processing steps to reach the functional form. Little is known about how the processing of miRNAs is modulated. Here we show that the RNA-binding protein DiGeorge critical region-8 (DGCR8), which is essential for the first processing step, is a heme-binding protein. The association with heme promotes dimerization of DGCR8. The heme-bound DGCR8 dimer seems to trimerize upon binding pri-miRNAs and is active in triggering pri-miRNA cleavage, whereas the heme-free monomer is much less active. A heme-binding region of DGCR8 inhibits the pri-miRNA–processing activity of the monomer. This putative autoinhibition is overcome by heme. Our finding that heme is involved in pri-miRNA processing suggests that the gene-regulation network of miRNAs and signal-transduction pathways involving heme might be connected.
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A clone containing human Drosha sequence was kindly provided by V.N. Kim (Seoul National University). We thank T. Cech, D. Eisenberg, J. Fukuto, C.W. Goulding and S. Kurtistani for comments of the manuscript, R. Landgraf, V. Kickhoefer and L. Simpson for assistance and advice on insect-cell protein expression, and D. Black, S. Clarke, A. Fire, A. Fulco, O. Hankinson, S. Merchant and J.S. Valentine for discussion. This work was supported in part by seed grants from the Stein Oppenheimer Endowment Fund and the Jonsson Comprehensive Cancer Center at UCLA to F.G. The UCLA Functional Proteomics Center was established and equipped by a grant from the W.M. Keck Foundation. J.A.L. acknowledges support from the UCLA–US Department of Energy Institute for Genomics and Proteomics, and the US National Institutes of Health (RR 20004).
The authors declare no competing financial interests.
Fluorescence excitation and emission spectra of NC1. (PDF 531 kb)
ESI mass spectrum of NC1 dimer. (PDF 642 kb)
Standard curves for SEC experiments. (PDF 783 kb)
Quantification of the heme cofactor bound to DGCR8. (PDF 802 kb)
SEC experiments of C352H and C430A. (PDF 714 kb)
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