The PAZ domain is a domain of about 110 amino-acids which has been named after three of the proteins containing it: Piwi, Argonaute and Zwille/Pinhead [1]. It is found in the Argonaute and the Dicer protein families, whose members are implicated in stem-cell fate determination and post-transcriptional gene silencing (PTGS) through RNA interference (RNAi) [2,3]. PTGS RNAi is the mechanism through which double-stranded RNAs (dsRNAs) silence cognate genes. It is characterized by the presence of RNAs of about 22 nucleotides in length that are homologous to the gene to be suppressed. These nucleotide sequences serve as guide sequences that instruct the RNA-induced silencing complex (RISC) nuclease to destroy specific mRNAs [4]. Proteins belonging to the Dicer family are enzymes that are involved in the generation of putative guide RNAs by specifically cleaving dsRNAs [3]. The function of proteins of the Argonaute family is not known, but they could be implicated in transcriptional initiation [2].

Although the sequence of the PAZ domain does not reveal its possible role, its presence in both the Argonaute and Dicer protein families indicates that it might be important for the function of these proteins in RNAi and cell differentiation [1].

Some proteins known to contain a PAZ domain are listed below:

Proteins of the Argonaute family:

• Eukaryotic translation initiation factor 2C (eIF2C). It plays an important role in the eukaryotic peptide chain initiation process.
• Drosophila Piwi proteins. It is essential for germ-line stem cell self- renewal.
• Caenorhabditis elegans RNA Interference Deficient 1 (rde-1). It is required for RNAi and transposon silencing.
• Arabidopsis thaliana Argonaute (Ago) protein. It is involved in RNAi as well as in leaves and floral organs development and axillary meristems formation.
• Arabidopsis thaliana Zwille (Zll) or Pinhead (Pnh). It is a developmental protein required for reliable formation of primary and axillary shoot apical meristems.

These proteins also contain a Piwi domain (see <PDOC50822>) in addition to the PAZ domain [1].

Proteins of the Dicer family:

• Drosophila Dicer (Dcr) protein.
• Arabidopsis thaliana Carpel Factory (CAF) protein. It is homologous to Dcr and plays a role in floral meristem determinacy [5].

In addition to the PAZ domain, these proteins contain DEAD/DEAH box helicase (see <PDOC00039>), RNase III (see <PDOC00448>) and double stranded RNA-binding domains (dsRBD) (see <PDOC50137>).

The profile we have developed covers the entire PAZ domain.