Piwi (or PIWI) were identified as gene regulation responsible for stem cell and germ cell differentiation.
Protein structure and function
The structure of several Piwi and
Argonaute (Ago) have been solved. Piwi proteins are RNA-binding proteins with 2 or 3
domains: The N-terminal
PAZ domain binds the 3'-end of the guide RNA; the middle
MID domain binds the 5'-phosphate of RNA; and the C-terminal
PIWI domain acts as an
Ribonuclease H endonuclease that can cleave RNA.
The small RNA partners of Ago proteins are
(miRNAs). Ago proteins utilize miRNAs to silence genes post-transcriptionally or use small-interfering RNAs (siRNAs) in both transcription and post-transcription silencing mechanisms. Piwi proteins interact with piRNAs (28–33 nucleotides) that are longer than miRNAs and siRNAs (~20 nucleotides), suggesting that their functions are distinct from those of Ago proteins.
Human Piwi proteins
Presently there are four known human Piwi proteins—PIWI-like protein 1, PIWI-like protein 2, PIWI-like protein 3 and PIWI-like protein 4. Human Piwi proteins all contain two RNA binding domains, PAZ and Piwi. The four PIWI-like proteins have a spacious binding site within the PAZ domain which allows them to bind the bulky 2’-OCH3 at the 3’ end of piwi-interacting RNA.
One of the major human homologues, whose upregulation is implicated in the formation of such as , is called hiwi (for human piwi ).
Homologous proteins in mice have been called miwi (for mouse piwi).
Role in germline cells
PIWI proteins play a crucial role in fertility and germline development across animals and ciliates. Recently identified as a polar granule component, PIWI proteins appear to control germ cell formation so much so that in the absence of PIWI proteins there is a significant decrease in germ cell formation. Similar observations were made with the mouse homologs of PIWI, MILI, MIWI and MIWI2. These homologs are known to be present in spermatogenesis. Miwi is expressed in various stages of spermatocyte formation and spermatid elongation where Miwi2 is expressed in
Sertoli cells. Mice deficient in either Mili or Miwi-2 have experienced spermatogenic stem cell arrest and those lacking Miwi-2 underwent a degradation of spermatogonia.
The effects of piwi proteins in human and mouse germlines seems to stem from their involvement in translation control as Piwi and the small noncoding RNA, piwi-interacting RNA (piRNA), have been known to co-fractionate polysomes. The piwi-piRNA pathway also induces
heterochromatin formation at
centromeres,
thus affecting transcription. The piwi-piRNA pathway also appears to protect the genome. First observed in Drosophila, mutant piwi-piRNA pathways led to a direct increase in dsDNA breaks in ovarian germ cells. The role of the piwi-piRNA pathway in transposon silencing may be responsible for the reduction in dsDNA breaks in germ cells.
Role in RNA interference
The
piwi domain is a
protein domain found in piwi proteins and a large number of related
nucleic acid-binding proteins, especially those that bind and cleave
RNA. The function of the domain is double stranded-RNA-guided hydrolysis of single stranded-RNA that has been determined in the
argonaute family of related proteins.
Argonautes, the most well-studied family of nucleic-acid binding proteins, are
RNase H-like
that carry out the
catalysis functions of the RNA-induced silencing complex (RISC). In the well-known cellular process of
RNA interference, the argonaute protein in the RISC complex can bind both
siRNA (siRNA) generated from
exogenous double-stranded RNA and
microRNA (miRNA) generated from
endogenous non-coding RNA, both produced by the
ribonuclease Dicer, to form an RNA-RISC complex. This complex binds and cleaves complementary
messenger RNA, destroying it and preventing its translation into protein. Crystallised piwi domains have a conserved basic
binding site for the 5' end of bound RNA; in the case of argonaute proteins binding siRNA strands, the last unpaired
nucleotide base of the siRNA is also stabilised by
base stacking-interactions between the base and neighbouring
tyrosine residues.
Recent evidence suggests that the functional role of piwi proteins in germ-line determination is due to their capacity to interact with miRNAs. Components of the miRNA pathway appear to be present in pole plasm and to play a key role in early development and morphogenesis of Drosophila melanogaster , in which germ-line maintenance has been extensively studied.
piRNAs and transposon silencing
A novel class of longer-than-average miRNAs known as Piwi-interacting RNAs (piRNAs) has been defined in
cells, about 26-31
long as compared to the more typical miRNA or siRNA of about 21 nucleotides. These piRNAs are expressed mainly in
spermatogenesis cells in the
testes of mammals.
But studies have reported that piRNA expression can be found in the ovarian somatic cells and neuron cells in invertebrates, as well as in many other mammalian somatic cells. piRNAs have been identified in the
of
mouse,
, and
, with an unusual "clustered" genomic organization
that may originate from repetitive regions of the genome such as
or regions normally organized into
heterochromatin, and which are normally derived exclusively from the
antisense strand of double-stranded RNA.
piRNAs have thus been classified as
repeat-associated small interfering RNAs (
).
Although their biogenesis is not yet well understood, piRNAs and Piwi proteins are thought to form an endogenous system for silencing the expression of selfish DNA such as retrotransposons and thus preventing the gene products of such sequences from interfering with germ cell formation.
Footnotes
External links
