InterPro domain: IPR032473

This domain (Mid) is found to be part of the Piwi-lobe of the argonaute proteins. It is composed of a parallel four-stranded beta-sheet core surrounded by four alpha-helices and two additional short alpha-helices. It most closely resembles the amino terminal tryptic core of the E.coli lactose repressor. There is an extensive interface between the Mid and the Piwi domains. The conserved C-terminal half or the Mid has extensive interactions with Piwi, with a deep basic pocket on the surface of the `Mid adjacent to the interface with Piwi. The Mid carries a binding pocket for the 5' phosphate overhang of the guide strand of DNA [ 1 ]. The N, Mid, and Piwi domains form a base upon which the PAZ domain sits, resembling a duck. The 5' phosphate and the U1 base are held in place by a conserved network of interactions from protein residues of the Mid and Piwi domains in order to place the guide uniquely in the proper position observed in all Argonaute-RNA complexes [ 2 ].

1. Crystal structure of A. aeolicus argonaute, a site-specific DNA-guided endoribonuclease, provides insights into RISC-mediated mRNA cleavage. Mol. Cell 19, 405-19
2. The making of a slicer: activation of human Argonaute-1. Cell Rep 3, 1901-9