Tudor staphylococcal nuclease is an evolutionarily conserved component of the programmed cell death degradome

Abstract

Programmed cell death (PCD) is executed by proteases, which cleave diverse proteins thus modulating their biochemical and cellular functions. Proteases of the caspase family and hundreds of caspase substrates constitute a major part of the PCD degradome in animals. Plants lack close homologues of caspases, but instead possess an ancestral family of cysteine proteases, metacaspases. Although metacaspases are essential for PCD, their natural substrates remain unknown. Here we show that metacaspase mcII-Pa cleaves a phylogenetically conserved protein, TSN (Tudor staphylococcal nuclease), during both developmental and stress-induced PCD. TSN knockdown leads to activation of ectopic cell death during reproduction, impairing plant fertility. Surprisingly, human TSN (also known as p100 or SND1), a multifunctional regulator of gene expression, is cleaved by caspase-3 during apoptosis. This cleavage impairs the ability of TSN to activate mRNA splicing, inhibits its ribonuclease activity and is important for the execution of apoptosis. Our results establish TSN as the first biological substrate of metacaspase and demonstrate that despite the divergence of plants and animals from a common ancestor about one billion years ago and their use of distinct PCD pathways, both have retained a common mechanism to compromise cell viability through the cleavage of the same substrate, TSN.

Antagonic activities of Trypanosoma cruzi metacaspases affect the balance between cell proliferation, death and differentiation

“…[16][17][18]28 All metacaspases studied so far are strictly specific for basic amino-acid residues at P1 position when using either synthetic peptides or protein as substrates. 9,22,29 However, significant differences were found when studying if protein maturation was required for activity. Plant type II, but not type I, metacaspases were clearly shown to be autocatalytically activated in vitro by a cleavage event at the p20-p10 boundary.…”

“…5 Their substrate specificity is for basic residues at P1 position, making them unable to cleave caspase substrates. 6 Evidences suggest that metacaspases modulate cell death, [7][8][9][10][11] cell cycle progression [12][13][14] and protein aggregation, 15 but there is still controversy about their functions and their relation with caspases. [16][17][18] The fact that metacaspases are not present in humans and fulfil important roles in trypanosomatids make them attractive drug targets, and a first series of inhibitors with trypanocidal activity has been developed recently.…”

Metacaspases are caspases. Doubt no more

“…Furthermore, caspase-3-dependent cleavage of HsTSN inhibited its ribonuclease activity and impaired its stimulatory function in pre-mRNA splicing. 11 The aforementioned experiments nicely illustrate how caspases and metacaspases can degrade similar deathrelevant proteins across phyla. However, differences certainly exist in the TSN-metacaspase/caspase system between plants and animals.…”

“…In addition, reverse genetics analyses carried out in Arabidopsis by crossing two transfer DNA knockout alleles for each of the two Arabidopsis TSN genes showed impairment of plant fertility and of embryogenesis (including a high incidence of reduced viable pollen and aborted ovule development). 11 In a further twist, Bozhkov and colleagues 11 revealed that TSN is involved in human PCD. Just as PaTSN is a substrate for the metacaspase mcII-Pa, human TSN (HsTSN), known to be a multifunctional regulator of gene expression, was cleaved by caspase-3 in vitro and in vivo.…”

Tudor Staphylococcal Nuclease plays two antagonistic roles in RNA metabolism under stress

“…15 The domain composition of TSN is also conserved, invariably comprising a tandem repeat of 4 non-canonical SN domains followed by a Tudor and a C-terminal partial SN domain. 16 In animals, TSN functions in several gene expression pathways in both the nucleus and the cytoplasm, including regulation of Figure 1. Colocalization analysis of GFP-DCP1 and eIF4E proteins.…”