T2 Family ribonucleases: ancient enzymes with diverse roles

Abstract

Ribonucleases of the T2 family are found in the genomes of protozoans, plants, bacteria, animals and viruses. A broad range of biological roles for these ribonucleases have been suggested, including scavenging of nucleic acids, degradation of self-RNA, serving as extra- or intracellular cytotoxins, and modulating host immune responses. Recently, RNaseT2 family members have been implicated in human pathologies such as cancer and parasitic diseases. Interestingly, certain functions of RNaseT2 family members are independent of their nuclease activity, suggesting that these proteins have additional functions. Moreover, humans lacking RNASET2 manifest a defect in neurological development, perhaps due to aberrant control of the immune system. We review the basic structure and function of RNaseT2 family members and their biological roles.

Figure 1. Structure and catalytic mechanism for T2 family ribonucleases

(a) 3D structure of fungal ribonuclease Rh (Rhizopus niveus) with conserved amino acid sequences (CASI and CASII) in dark red (I) and light red (II) [13]. Nucleotide binding sites (B1 and B2) are indicated by yellow (B1) and blue (B2) colors, and overlap with active site regions is indicated by orange (B1) and purple (B2) (regions mapped according to previous analyses [3, 17]). Histidines involved in acid-base catalysis are labeled [2, 3]. (b) Mechanism for transferase type RNase catalysis. Transphosphorylation and hydrolysis steps are shown. In the first step, one histidine acts as an acid (H46, as numbered for Rh) with the other acting as a base (H109, as numbered for Rh), generating a 2′–3′ cyclic phosphate intermediate. These histidines reverse roles in acid-base catalysis in the second step (hydrolysis) to produce oligo- or mononucleotides with a terminal 3′-phosphate. Composed from mechanism described in Kurihara et al., [13].

Figure 2. Diverse biological roles for T2 family ribonucleases

Although T2 ribonucleases exhibit a conserved core structure, these enzymes perform diverse functions. (a) Scavenging of extracellular or intracellular RNAs. Extracellular RNA. Plants secrete RNaseT2 proteins (red), such as tomato RNase LE, during phosphate starvation; the T2 proteins might scavenge extracellular RNA and recycle RNA components to the cell [–39]. Intracellular RNA. During autophagy, protein aggregates are internalized by autophagosomes and taken up by the vacuole/lysosome [41, 43]. Intracellular RNA, as part of ribonucleoprotein complexes (RNPs; blue) such as P-granules [43], might be degraded in this manner by RNaseT2 proteins within the vacuole/lysosome. (b)Plant S-RNases and coordination of self-incompatibility. A model for compartmentalized control of S-RNase activity is depicted. S-RNases are secreted by pistil cells to the extracellular matrix (ECM) wherein they can be internalized to the cytosol of the pollen tube and have been shown to localize to a vacuole-like compartment [5, 59, 60, 75]. This compartment is maintained if pollen is compatible, but disintegration of the compartment occurs late in incompatible pollination, perhaps enabling the degradation of pollen RNAs and blocking pollen tube development [5]. Pistil factors required for decompartmentalization and the pollen protein SLF are not shown for simplicity. (c) Pestiviral envelope glycoprotein, E^rns, and cytotoxicity. The E^rns protein found in pestiviruses is secreted by its host’s infected cells and internalized by host immune cells in a receptor-independent manner through its positive charge (C-terminus) [31, 51]. There, it depletes host immune cells through both catalytic-dependent and –independent functions [–56]. (d) The budding yeast RNaseT2 protein, Rny1, and the oxidative stress response. Rny1 localizes to the vacuole in unstressed cells, and oxidative stress causes its release into the cytoplasm where it cleaves tRNAs and rRNAs [4, 67]. Rny1 also possesses a catalytic-independent activity which regulates cellular viability during oxidative stress [4]. ER: endoplasmic reticulum