PIWI-interacting RNAs (piRNAs) silence transposons to maintain genome integrity in animal germ lines. piRNAs are classified as primary and secondary piRNAs, depending on their biogenesis machinery. Primary piRNAs are processed from long non-coding RNA precursors transcribed from piRNA clusters in the genome through the primary processing pathway. Although the existence of a ribonuclease participating in this pathway has been predicted, its molecular identity remained unknown. Here we show that Zucchini (Zuc), a mitochondrial phospholipase D (PLD) superfamily member, is an endoribonuclease essential for primary piRNA biogenesis. We solved the crystal structure of Drosophila melanogaster Zuc (DmZuc) at 1.75 Å resolution. The structure revealed that DmZuc has a positively charged, narrow catalytic groove at the dimer interface, which could accommodate a single-stranded, but not a double-stranded, RNA. DmZuc and the mouse homologue MmZuc (also known as Pld6 and MitoPLD) showed endoribonuclease activity for single-stranded RNAs in vitro. The RNA cleavage products bear a 5'-monophosphate group, a hallmark of mature piRNAs. Mutational analyses revealed that the conserved active-site residues of DmZuc are critical for the ribonuclease activity in vitro, and for piRNA maturation and transposon silencing in vivo. We propose a model for piRNA biogenesis in animal germ lines, in which the Zuc endoribonuclease has a key role in primary piRNA maturation.

PIWI-clade Argonaute proteins associate with PIWI-interacting RNAs (piRNAs) and silence transposable elements in animal gonads. Here, we report the crystal structure of a silkworm PIWI-clade Argonaute, Siwi, bound to the endogenous piRNA, at 2.4 Å resolution. Siwi adopts a bilobed architecture consisting of N-PAZ and MID-PIWI lobes, in which the 5' and 3' ends of the bound piRNA are anchored by the MID-PIWI and PAZ domains, respectively. A structural comparison of Siwi with AGO-clade Argonautes reveals notable differences in their nucleic-acid-binding channels, likely reflecting the distinct lengths of their guide RNAs and their mechanistic differences in guide RNA loading and cleavage product release. In addition, the structure reveals that Siwi and prokaryotic, but not eukaryotic, AGO-clade Argonautes share unexpected similarities, such as metal-dependent 5'-phosphate recognition and a potential structural transition during the catalytic-tetrad formation. Overall, this study provides a critical starting point toward a mechanistic understanding of piRNA-mediated transposon silencing.

Killer cell lectin-like receptor G1 (KLRG1) is an inhibitory receptor expressed on subsets of natural killer (NK) cells and T cells, for which no endogenous ligands are known. Here, we show that KLRG1 binds three of the classical cadherins (E-, N-, and R-), which are ubiquitously expressed in vertebrates and mediate cell–cell adhesion by homotypic or heterotypic interactions. By expression cloning using the mouse KLRG1 tetramer as a probe, we identified human E-cadherin as a xenogeneic ligand. We also identified a syngeneic interaction between mouse KLRG1 and mouse E-cadherin. Furthermore, we show that KLRG1 binds N- and R-cadherins. Finally, we demonstrate that E-cadherin binding of KLRG1 prevents the lysis of E-cadherin–expressing targets by KLRG1+ NK cells. These results suggest that KLRG1 ligation by E-, N-, or R-cadherins may regulate the cytotoxicity of killer cells to prevent damage to tissues expressing the cadherins.

Acyl-CoA:glycerol-3-phosphate acyltransferase (GPAT) and acyl-CoA: 1-acyl-glycerol-3-phosphate acyltransferase (AGPAT) are involved in the de novo synthesis of triacylglycerol (TAG) and glycerophospholipids. Many enzymes belonging to the GPAT/AGPAT family have recently been identified and their physiological or pathophysiological roles have been proposed. The roles of GPAT/AGPAT in the synthesis of TAG and obesity-related diseases were revealed through the identification of causative genes of these diseases or analyses of genetically manipulated animals. Recent studies have suggested that some isoforms of GPAT/AGPAT family enzymes are involved in the fatty acid remodeling of phospholipids. The enzymology of GPAT/AGPAT and their physiological/pathological roles in the metabolism of glycerolipids have been described and discussed in this review.

Natural killer (NK) cells express receptors that recognize major histocompatibility complex (MHC) class I molecules and regulate cytotoxicity of target cells. In this study, we demonstrate that Ly49A, a prototypical C-type lectin–like receptor expressed on mouse NK cells, requires species-specific determinants on β2-microglobulin (β2m) to recognize its mouse MHC class I ligand, H-2Dd. The involvement of β2m in the interaction between Ly49A and H-2Dd is also demonstrated by the functional effects of a β2m-specific antibody. We also define three residues in α1/α2 and α3 domains of H-2Dd that are critical for the recognition of H-2Dd on target cells by Ly49A. In the crystal structure of the Ly49A/H-2Dd complex, these residues are involved in hydrogen bonding to Ly49A in one of the two potential Ly49A binding sites on H-2Dd. These data unambiguously indicate that the functional effect of Ly49A as an MHC class I–specific NK cell receptor is mediated by binding to a concave region formed by three structural domains of H-2Dd, which partially overlaps the CD8 binding site.