Drosha is a Class 2 ribonuclease III enzyme [5] that in humans is encoded by the DROSHA (formerly RNASEN) gene. [6] [7] [8] It is the primary nuclease that executes the initiation step of miRNA processing in the nucleus. It works closely with DGCR8 and in correlation with Dicer. It has been found significant in clinical knowledge for cancer prognosis. [9] and HIV-1 replication. [10]
Human Drosha was cloned in 2000 when it was identified as a nuclear dsRNA ribonuclease involved in the processing of ribosomal RNA precursors. [11] The other two human enzymes that participate in the processing and activity of miRNA are the Dicer and Argonaute proteins. Recently, proteins like Drosha have been found significant in cancer prognosis [9] and HIV-1 replication. [10]
Members of the ribonuclease III superfamily of double-stranded (ds) RNA-specific endoribonucleases participate in diverse RNA maturation and decay pathways in eukaryotic and prokaryotic cells. [12] The RNase III Drosha is the core nuclease that executes the initiation step of microRNA (miRNA) processing in the nucleus. [8] [11]
The microRNAs thus generated are short RNA molecules that regulate a wide variety of other genes by interacting with the RNA-induced silencing complex (RISC) to induce cleavage of complementary messenger RNA (mRNA) as part of the RNA interference pathway. MicroRNA molecules are synthesized as long RNA primary transcripts known as a pri-miRNAs, which are cleaved by Drosha to produce a characteristic stem-loop structure of about 70 base pairs long, known as a pre-miRNA. [11] Pre-miRNAs, when associated with EXP5, are stabilized due to removal of the 5' cap and 3' poly(A) tail. [13] </ref> Drosha exists as part of a protein complex called the Microprocessor complex, which also contains the double-stranded RNA binding protein DGCR8 (called Pasha in D. melanogaster and C. elegans ). [14] DGCR8 is essential for Drosha activity and is capable of binding single-stranded fragments of the pri-miRNA that are required for proper processing. [15] The Drosha complex also contains several auxiliary factors such as EWSR1, FUS, hnRNPs, p68, and p72. [16]
Both Drosha and DGCR8 are localized to the cell nucleus, where processing of pri-miRNA to pre-miRNA occurs. These two proteins homeostatically control miRNA biogenesis by an auto-feedback loop. [16] A 2nt 3' overhang is generated by Drosha in the nucleus recognized by Dicer in the cytoplasm, which couples the upstream and downstream processing events. Pre-miRNA is then further processed by the RNase Dicer into mature miRNAs in the cell cytoplasm. [11] [16] There also exists an isoform of Drosha that does not contain a nuclear localization signal, which results in the generation of c-Drosha. [17] [18] This variant has been shown to localize to the cell cytoplasm rather than the nucleus, but the effects on pri-miRNA processing are yet unclear.
Both Drosha and Dicer also participate in the DNA damage response. [19]
Certain miRNAs have been found to deviate from conventional biogenesis pathways and do not necessarily require Drosha or Dicer, which is because they do not require the processing of pri-miRNA to pre-miRNA. [16] Drosha-independent miRNAs derive from mirtrons, which are genes that encode for miRNAs in their introns and make use of splicing to bypass Drosha cleavage. Simtrons are mirtron-like, splicing-independent, and do require Drosha mediated cleavage, although they do not require most proteins in the canonical pathway such as DGCR8 or Dicer. [10]
Drosha and other miRNA processing enzymes may be important in cancer prognosis. [9] Both Drosha and Dicer can function as master regulators of miRNA processing and have been observed to be down-regulated in some types of breast cancer. [20] The alternative splicing patterns of Drosha in The Cancer Genome Atlas have also indicated that c-drosha appears to be enriched in various types of breast cancer, colon cancer, and esophageal cancer. [18] However, the exact nature of the association between microRNA processing and tumorigenesis is unclear, [21] but its function can be effectively examined by siRNA knockdown based on an independent validation. [22]
Drosha and other miRNA processing enzymes may also be important in HIV-1 replication. miRNAs contribute to the innate antiviral defense. This can be shown by the knockdown of two important miRNA processing proteins, Drosha and Dicer, which leads to a significant enhancement of viral replication in PBMCs from HIV-1-infected patients. Thus, Drosha, in conjunction with Dicer, seem to have a role in controlling HIV-1 replication. [10]
MicroRNA (miRNA) are small, single-stranded, non-coding RNA molecules containing 21 to 23 nucleotides. Found in plants, animals and some viruses, miRNAs are involved in RNA silencing and post-transcriptional regulation of gene expression. miRNAs base-pair to complementary sequences in mRNA molecules, then silence said mRNA molecules by one or more of the following processes:
A non-coding RNA (ncRNA) is a functional RNA molecule that is not translated into a protein. The DNA sequence from which a functional non-coding RNA is transcribed is often called an RNA gene. Abundant and functionally important types of non-coding RNAs include transfer RNAs (tRNAs) and ribosomal RNAs (rRNAs), as well as small RNAs such as microRNAs, siRNAs, piRNAs, snoRNAs, snRNAs, exRNAs, scaRNAs and the long ncRNAs such as Xist and HOTAIR.
Ribonuclease H is a family of non-sequence-specific endonuclease enzymes that catalyze the cleavage of RNA in an RNA/DNA substrate via a hydrolytic mechanism. Members of the RNase H family can be found in nearly all organisms, from bacteria to archaea to eukaryotes.
Dicer, also known as endoribonuclease Dicer or helicase with RNase motif, is an enzyme that in humans is encoded by the DICER1 gene. Being part of the RNase III family, Dicer cleaves double-stranded RNA (dsRNA) and pre-microRNA (pre-miRNA) into short double-stranded RNA fragments called small interfering RNA and microRNA, respectively. These fragments are approximately 20–25 base pairs long with a two-base overhang on the 3′-end. Dicer facilitates the activation of the RNA-induced silencing complex (RISC), which is essential for RNA interference. RISC has a catalytic component Argonaute, which is an endonuclease capable of degrading messenger RNA (mRNA).
Ribonuclease P is a type of ribonuclease which cleaves RNA. RNase P is unique from other RNases in that it is a ribozyme – a ribonucleic acid that acts as a catalyst in the same way that a protein-based enzyme would. Its function is to cleave off an extra, or precursor, sequence of RNA on tRNA molecules. Further, RNase P is one of two known multiple turnover ribozymes in nature, the discovery of which earned Sidney Altman and Thomas Cech the Nobel Prize in Chemistry in 1989: in the 1970s, Altman discovered the existence of precursor tRNA with flanking sequences and was the first to characterize RNase P and its activity in processing of the 5' leader sequence of precursor tRNA. Recent findings also reveal that RNase P has a new function. It has been shown that human nuclear RNase P is required for the normal and efficient transcription of various small noncoding RNAs, such as tRNA, 5S rRNA, SRP RNA and U6 snRNA genes, which are transcribed by RNA polymerase III, one of three major nuclear RNA polymerases in human cells.
Ribonuclease III (RNase III or RNase C)(BRENDA 3.1.26.3) is a type of ribonuclease that recognizes dsRNA and cleaves it at specific targeted locations to transform them into mature RNAs. These enzymes are a group of endoribonucleases that are characterized by their ribonuclease domain, which is labelled the RNase III domain. They are ubiquitous compounds in the cell and play a major role in pathways such as RNA precursor synthesis, RNA Silencing, and the pnp autoregulatory mechanism.
The microprocessor complex subunit DGCR8(DiGeorge syndrome critical region 8) is a protein that in humans is encoded by the DGCR8 gene. In other animals, particularly the common model organisms Drosophila melanogaster and Caenorhabditis elegans, the protein is known as Pasha. It is a required component of the RNA interference pathway.
MicroRNA 187 is a protein that in humans is encoded by the MIR187 gene.
The microprocessor complex is a protein complex involved in the early stages of processing microRNA (miRNA) and RNA interference (RNAi) in animal cells. The complex is minimally composed of the ribonuclease enzyme Drosha and the dimeric RNA-binding protein DGCR8, and cleaves primary miRNA substrates to pre-miRNA in the cell nucleus. Microprocessor is also the smaller of the two multi-protein complexes that contain human Drosha.
DCL1 is a gene in plants that codes for the DCL1 protein, a ribonuclease III enzyme involved in processing double-stranded RNA (dsRNA) and microRNA (miRNA). Although DCL1, also called Endoribonuclease Dicer homolog 1, is named for its homology with the metazoan protein Dicer, its role in miRNA biogenesis is somewhat different, due to substantial differences in miRNA maturation processes between plants and animals, as well due to additional downstream plant-specific pathways, where DCL1 paralogs like DCL4 participate, such Trans-acting siRNA biogenesis.
MicroRNA 203a is a small RNA that in humans is encoded by the preMIR203A gene.
MicroRNA 148a is a miRNA that in humans is encoded by the MIR148A gene.
MicroRNA let-7f-2 is a protein that in humans is encoded by the MIRLET7F2 gene.
MicroRNA 196b is a protein that in humans is encoded by the MIR196B gene.
MicroRNA 495 is a protein that in humans is encoded by the MIR495 gene.
MicroRNA 141 is a non-coding RNA molecule that in humans is encoded by the MIR141 gene.
MicroRNA 375 is a non coding RNA that in humans is encoded by the MIR375 gene.
MicroRNA 195 is a microRNA that in humans is encoded by the MIR195 gene.
MicroRNA 885 is a protein that in humans is encoded by the MIR885 gene.
MicroRNA 124-3 is a protein that in humans is encoded by the MIR124-3 gene.
