The microprocessor complex, composed of the Drosha ribonuclease and the DGCR8 protein, plays a crucial role in initiating the precise production of microRNAs (miRNAs). Here's an overview of the steps involved in this process:
1. Transcription: Primary miRNA (pri-miRNA) transcripts are produced by RNA polymerase II in the nucleus. These pri-miRNAs are long RNA molecules that contain the sequences of mature miRNAs.
2. Binding of the microprocessor complex: The microprocessor complex recognizes and binds to specific regions within the pri-miRNA transcript. DGCR8, a double-stranded RNA-binding protein, initially binds to the pri-miRNA and recruits Drosha to the complex.
3. Cleavage by Drosha: Drosha, an RNase III enzyme, precisely cleaves the pri-miRNA at specific sites, generating a smaller precursor miRNA (pre-miRNA). This cleavage typically occurs approximately 11 nucleotides (nt) away from the base of the terminal loop of the pri-miRNA.
4. Structural features: The pre-miRNA is a ~70-nt RNA molecule with a characteristic hairpin structure. It consists of a double-stranded RNA stem region and a loop region.
5. Export from the nucleus: The pre-miRNA is exported from the nucleus to the cytoplasm through the exportin-5 (EXP5) protein. EXP5 recognizes and binds to the pre-miRNA and transports it across the nuclear membrane.
6. Further processing: Once in the cytoplasm, the pre-miRNA is further processed by the Dicer enzyme, which cleaves the hairpin structure to produce a short, double-stranded miRNA duplex.
7. miRNA loading into RISC: One strand of the miRNA duplex (the mature miRNA) is incorporated into the RNA-induced silencing complex (RISC). RISC, which includes the Argonaute (AGO) protein, then uses the mature miRNA as a guide to regulate gene expression by binding to complementary sequences in target messenger RNAs (mRNAs), leading to mRNA degradation or translational repression.
By precisely cleaving the pri-miRNA, the microprocessor complex plays a critical role in ensuring the accurate production of mature miRNAs and subsequent regulation of gene expression. Dysregulation of the microprocessor complex or its components can impact miRNA biogenesis and contribute to various diseases, including cancer and neurological disorders.
