The RISC Complex
What is the RISC complex?
-The active component in RNA Interference by regulating gene expression using miRNA regulated post-transcriptional suppression
-The RISC complex is a multi-protein complex that when activated, binds complementary mRNA and degrades it, consequently decreasing levels of protein translation causing a specific gene to be “turned off”
-The RISC complex is important in gene regulation and defense against viral infections by using siRNA to target specific mRNA.
miRNA has a nearly identical mechanism
What proteins are associated with the RISC complex?
DICER
-an endoribonuclease
-contains two RNaseIII domains and one Paz domain
-also contains the Dcr-2 and R2D2 domains
TRBP
-binds to the DICER complex in order to recruit the Ago2 protein
AGO2
-contained within the RISC complex
-contains two domains:
PIWI: The PIWI domain is a conserved domain within the Argonaute family.
The C-Terminus of this domain is required for endonuclease activity.
PAZ: The PAZ domain is also a conserved domein within the Argonaute family.
It is located near the center of the AGO2 protein and provides grooves substrate binding.
How does the RISC complex assemble and bind?
RNA Interference (RNAi) is initiated by DICER an RNase III Enzyme, which uses an RNA endonuclease activity to cleave double stranded RNA into 20-23 nt fragments known as miRNA. This cleavage is important because it provides 3’overhangs on each strand of the miRNA.
miRNA binds to a heterodimer of R2D2 and Dcr-2 proteins. Once bound the R2D2/Dcr-2 initiator complex is formed.
The 5’end of the RNA interacts with Dcr-2 forming the pre-RISC complex, which is the inactive form.
TRBP is an integral component of a Dicer-containing complex. TRBP is required to recruit Ago2 protein to interact with the bound miRNA.
Ago2 protein actives the pre-RISC complex, converting it into holo-RISC by binding to the double stranded miRNA, which is double stranded. Once Ago2 is bound, it degrades one strand from the RNA (the passenger strand), while selectively retaining the guide strand.
The holo-RISC complex recognizes mRNA with sequences complementary to the guide strand and binds to them.
Once mRNA is bound, it is degraded.
What diseases are caused by mutations in the RISC complex?
Wilms' tumors: Proteins similar to the Argonaute family reside in tandem on chromosome 1 (1p34-35). At the appearance of Wilms' tumors, this region is often lost. Wilms' tumors cause renal cancer in children and are the most common form of pediatric cancer.
The human PIWI domain is located on chromosome 12q.24.33.
Overexpression of this region of the chromosome have been linked to the development of testicular germ cell tumors.
The loss of this region of the chromosome correlates with ambiguous genitalia and hypogonadism.
How is the RISC complex used for gene therapy?
-Utilizing siRNA, the RISC complex as part of RNAi is currently being researched for theraputic applications in gene therapy:
-shut down viruses such as HIV and hepatitus
-target growth factor receptors in order to treat various cancers such as brain cancer
-target mRNA encoding for the HD allele that causes Huntingtons disease
-target mRNA causing macular degeneration
-further research is currently being conducted in order to expand the uses of RNAi gene therapy
Comments (1)
Christopher Korey said
at 9:07 am on Apr 7, 2009
Looks good. You are pretty far along. As you convert the outline to text, Remember to be concise about each subsection and provide link outs to other pages or papers that provide more in depth detail if that is required. If someone wants more information give them a way to find it not necessarily put it on the page. For example, describe each disease and then link out to a site like OMIM to provide detailed information. Try to divide the sections by inserting a horizontal bar. Remember to reference just like any other paper, images as well. Also, make the references links with the URLs that you have listed rather than pasting in the URL.
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