Structural basis for overhang-specific small interfering RNA recognition by the PAZ domain

Short RNAs mediate gene silencing, a process associated with virus resistance, developmental control and heterochromatin formation in eukaryotes(1-5). RNA silencing is initiated through Dicer-mediated processing of double-stranded RNA into small interfering RNA (siRNA)(6,7). The siRNA guide strand associates with the Argonaute protein in silencing effector complexes, recognizes complementary sequences and targets them for silencing(8-11). The PAZ domain is an RNA-binding module found in Argonaute and some Dicer proteins and its structure has been determined in the free state(12-14). Here, we report the 2.6 Angstrom crystal structure of the PAZ domain from human Argonaute eIF2c1 bound to both ends of a 9-mer siRNA-like duplex. In a sequence-independent manner, PAZ anchors the 2-nucleotide 3' overhang of the siRNA-like duplex within a highly conserved binding pocket, and secures the duplex by binding the 7-nucleotide phosphodiester backbone of the overhang-containing strand and capping the 5'-terminal residue of the complementary strand. On the basis of the structure and on binding assays, we propose that PAZ might serve as an siRNA-end-binding module for siRNA transfer in the RNA silencing pathway, and as an anchoring site for the 3' end of guide RNA within silencing effector complexes.