Emerging biological functions of ribonuclease 1 and angiogenin

Abstract

Pancreatic-type ribonucleases (ptRNases) are a large family of vertebrate-specific secretory endoribonucleases. These enzymes catalyze the degradation of many RNA substrates and thereby mediate a variety of biological functions. Though the homology of ptRNases has informed biochemical characterization and evolutionary analyses, the understanding of their biological roles is incomplete. Here, we review the functions of two ptRNases: RNase 1 and angiogenin. RNase 1, which is an abundant ptRNase with high catalytic activity, has newly discovered roles in inflammation and blood coagulation. Angiogenin, which promotes neovascularization, is now known to play roles in the progression of cancer and amyotrophic lateral sclerosis, as well as in the cellular stress response. Ongoing work is illuminating the biology of these and other ptRNases.

Keywords: Blood coagulation; endoribonuclease; extracellular RNA; inflammation; pancreatic-type ribonuclease; stress.

Figure 1.

Ribonucleases function at the crossroads between transcription and translation.

Figure 2.

Structures of human RNase 1 (UniProtKB P07998), angiogenin (UniProtKB P03950), and RI (UnitProtKB P13489). (A) Ribbon diagram of RNase 1. The active-site residues (His12, Lys41, and His119; circled), four disulfide bonds, and three sites of asparagine glycosylation (Asn34, Asn76, and Asn88) are shown explicitly with CPK colors. Right, surface charges (blue, positive; red, negative). The cationic binding cleft for RNA is apparent across the middle of the structure. (B) Ribbon diagram of angiogenin. The active-site residues (His13, Lys40, and His114; circled), three disulfide bonds, and three sites of serine phosphorylation (Ser28, Ser37, and Ser87) are shown explicitly. Right, surface charges. (C) Surface charges of RI, which is highly anionic. Images were made with the program PyMOL from Schrödinger (New York, NY). Atomic coordinates: RNase 1 and RI, PDB entry 1z7x (Johnson et al., 2007); angiogenin, PDB entry 1ang (Acharya et al., 1994).

Figure 3.

Putative mechanism for catalysis of RNA cleavage by ptRNases, highlighting the roles of the three active-site residues (green) depicted in Figure 2 (Findlay et al., 1961; Cuchillo et al., 2011). The 2′,3′-cyclic phosphodiester product can be hydrolyzed by ptRNases in a separate step (Cuchillo et al., 1993; Thompson et al., 1994).

Figure 4.

Domain swapping in the BS-RNase dimer (Kim et al., 1995a). The active-site residues (His12, Lys41, and His119) and six disulfide bonds are shown explicitly. Images were made with the program PyMOL and PDB entries 3bcm and 3bco (Merlino et al., 2008).

Figure 5.

Schematic of RNase 1 and angiogenin activities and substrates. RNase 1 acts primarily on targets outside of cells, where it degrades eRNAs. These eRNAs would otherwise act as activators of coagulation and inflammatory pathways. Angiogenin, in contrast, mediates its effects largely within cells, driving both cellular quiescence in response to stress via tRNA degradation and proliferation via pRNA degradation.

Figure 6.

Phylogenetic tree of homologous ptRNases expressed in vertebrate species, with annotations highlighting the co-emergence of coagulation factors in given groups of species. Protein sequences of RNase 1 homologs from UniProtKB P07998 (human), P00683 (mouse), P00684 (rat), P61823 (cow), P30374 (chicken), P80287 (iguana), A0A151LY34 (alligator), P11916 (bullfrog), Q8UVX5_LITPI (Northern leopard frog), H9GD73_ANOCA (anole), A0A1S3RRZ8 (salmon), and A5HAK0 (zebrafish) were used to generate a phylogenetic tree based on protein sequence similarity with the program phylogeny.fr (Castresana, 2000; Edgar, 2004; Chevenet et al., 2006; Anisimova and Gascuel, 2006; Guindon et al., 2010). The branch support value (red) is shown for each junction, and the number of substitutions per site is represented by line length. Line thickness at the termini of each leaf represents the number of ptRNase proteins reported to exist in each organism (Zhao et al., 1994; Irie et al., 1998; Beintema and Kleineidam, 1998; Cho et al., 2005; Pizzo et al., 2006; Kazakou et al., 2008; Pizzo et al., 2008). Species without identified ptRNases are represented by dashed lines that are not to scale; these species are included to highlight the emergence of vertebrate coagulation factors. The emergence of angiogenins in fish, RNase 1-like proteins in amphibians, and RNA-sensitive coagulation factors (FXII, FXI, and prekallikrein [PK]) in amphibians and mammals is highlighted at the appropriate junctions (Doolittle and Surgenor, 1962; Doolittle, 2009).