Presented by :Sasmita BehuraMBT – IRoll No - 1361024

PI-RNA

The discovery of PI-RNAs

In male flies, small RNAs derived from the supressor of stellate locus on the y-chromosome silence the x-linked stellate locus during spermatogenesis.

In su (ste) mutants , small RNAs targeting stellate are lost causing stellate protein crystals to form in the primary spermatocytes and reduced fertility .

Subsequent small RNA profiling studies in Drosophila reveal that su(ste) small RNAs only represent a small subset of a class of small RNAs with a size range 24-26nt and named as repeat associated small interfering RNAs (rasiRNAs).

And in mammals it has been seen that piwi protein associate with a class of endogenous RNAs termed as Pi-RNAs.

Thus Droshophila rasiRNAs were renamed as Pi-RNAs .

Piwi- interacting RNAs(knowns of the unknown)

• Piwi - interacting RNA is the largest class of small non-coding RNA molecules expressed in both vertebrates and invertebrates.

• Piwi- proteins are predominantly found in germline cells and associate with a novel class of small RNAs known as pi-RNAs.

• Different types of piwi- proteins are found i.e MILI, MIWI1 and MIWI2 in mice, ZIWI1 and ZIWI2 in zebra fish and Ago3 and Aub in mammals.

• Piwi is a class of gene was originally identified as encoding regulatory proteins responsible for maintaining the stability of cell division in germline cells.

Location of pi-rnaPi-RNAs are mainly found in clusters through out the

genomes and they may contains 10 up-to many thousands of pi- RNAs and piwi protein mainly found in germ cell and gonodial cell.

It is found in both mono-directional and bidirectional clusters which length is up to 20-1000 kilobases with 24-33 nuleotides.

BiogenesisExact biogenetic process of pi-RNA is not yet fully

understood. But there is a” ping-pong”-mechanism which gives an overall view about its biogenesis.

Pi-RNAs involved in the ping-pong cycle directs its attack on transposon transcripts and this cycle only acts at the level of transcription.

Pi-RNA production is a dicer- independent process.

In c . elegans pi-RNAs have a 5’- mono-phosphate and a 3’ modification that acts to block either 2’ or 3’ oxygen but the reason behind this modification isn’t clear. It may be there to increase pi-RNAs stability.

Ping-Pong mechanismPi-RNAs consists of 2-strands i.e sense strand and

antisense strand.

Ago3(argonaute protein) bound to sense strand pi-RNAs and catalyzes antisense strand cleavage at an A:U base-pair that generate the 5’ end of antisense strand.

Then 5’end of the resulting cleavage associate with Aub or piwi which generate mature 23-30nt antisense pi-RNAs.

Then the mature antisense pi-RNA argonaute complexes are then cleaved sense strand pi-RNA which associate with Ago3.

Then production of mature sense strand complete the cycle.

3’ methylation occures at both antisense and sense strand which is carried out by Hen1 RNA methyl transferse.

Mutation in Drosophila Hen1 reduces the length of pi-RNAs.

In Drosophila some genes like armitage and spindle E encodes putative helicase that required for pi-RNAs production.

As antisense pi-RNAs guide Aub or piwi to cleave the transposon m-RNAs and trigger the production of new antisense pi-RNAs.

So Ping-Pong model proposes a target dependent amplification of antisense pi-RNAs which depends on Ago3.

Compartmentalization of pi-RNAsActually most pi-RNA pathway proteins, localizes almost

exclusively to nurse cell nuclei so that pi-RNA production and function might be compartmentalized.

piwi protein family involves piwi,Aub and Ago3 which are found in nuage/cytoplasm , which is an amorphous electron dense cloud that surrounds the nurse cell nuclei.

Pi-RNA argonaute complexes appear to be catalytically active effectors and these localization studies thus suggest that piwi mediates nuclear functions for the pi-RNA pathway, where as Ago3 and Aub drive cytoplasmic funcion.

Localization of piwi class Argonautes in the Drosophila ovary

Model for compartmentalized production and function of piRNAs

Types of pi-RNAs in mammals

A,B- pi-RNA complex.

Argonaute protein

A- Eukaryotic ago protein with 4-domains.A crystal structure of ago protein in

Pyrococcus furiosus.

FunctionsPi-RNA complexes linked to both epigenetic ,post –translational

gene silencing and maintain germline DNA integrity.

Pi-RNAs also regulate the expression of protein coding genes.

Piwi target RNA cleavage by silencing.

Piwi required for the self renewing division of germline cells during oogenesis.

Mutation in any members of pi-RNA pathway causes derepression of transposons and also leads to the loss of fertility.

Piwi – complexes are imported to nucleus and mediate heterochromatin assembly.

Unknown factors about pi-RNAs It is still unknown that how a new pi-RNA response is

initiated and why some regions of our genome are so uniquely poised to proudce pi-RNAs.

The transcriptional region of pi-RNA cluster is still unknown.

There are also various proteins rather than piwi- family which are involves in ping-pong mechanism but there mode of action is also not understood.