Two small enzyme isoforms mediate mammalian mitochondrial poly(ADP-ribose) glycohydrolase (PARG) activity

Abstract

Poly(ADP-ribose)glycohydrolase (PARG) is the major enzyme capable of rapidly hydrolyzing poly (ADP-ribose)(PAR) formed by the diverse members of the PARP enzyme family. This study presents an alternative splice mechanism by which two novel PARG protein isoforms of 60 kDa and 55 kDa are expressed from the human PARG gene, termed hPARG60 and hPARG55, respectively. Homologous forms were found in the mouse (mPARG63 and mPARG58) supporting the hypothesis that expression of small PARG isoforms is conserved among mammals. A PARG protein of ∼60kDa has been described for decades but with its genetic basis unknown, it was hypothesized to be a product of posttranslational cleavage of larger PARG isoforms. While this is not excluded entirely, isolation and expression of cDNA clones from different sources of RNA indicate that alternative splicing leads to expression of a catalytically active hPARG60 in multiple cell compartments. A second enzyme, hPARG55 that can be expressed through alternative translation initiation from hPARG60 transcripts is strictly targeted to the mitochondria. Functional studies of a mitochondrial targeting signal (MTS) in PARG exon IV suggest that hPARG60 may be capable of shuttling between nucleus and mitochondria, which would be in line with a proposed function of PAR in genotoxic stress-dependent, nuclear-mitochondrial crosstalk.

Radiation-induced mitotic catastrophe in PARG-deficient cells

“…PAR is rapidly degraded by poly(ADP-ribose) glycohydrolase (PARG), an enzyme with both exo-and endoglycosidase activities that generate large amounts of free ADP-ribose. In contrast to the 17 members of the human PARP family Schreiber et al, 2006), human PARG is encoded by a single gene, but is present within the cell through different isoforms displaying various subcellular localizations: nuclear (PARG 111 ), cytoplasmic (PARG 102 and PARG 99 ) and mitochondrial (PARG 60 ) (Meyer et al, 2007). Study of the functional role of PARG through the generation of mutant mouse models has been hampered by the existence of these isoforms.…”

ADP-ribosyl-acceptor hydrolase 3 regulates poly (ADP-ribose) degradation and cell death during oxidative stress

“…Unlike the multigene PARP family, the single parg gene, via alternative splicing, gives rise to isoforms with different subcellular localizations and activities (18). Full-length, 110-kDa PARG is found in the nucleus, with other forms in the cytoplasm and mitochondria (19)(20)(21). In contrast to that of PARP1, the action of PARG in response to DNA damage is still controversial.…”

Functional Localization of Two Poly(ADP-Ribose)-Degrading Enzymes to the Mitochondrial Matrix

“…At least larger polymers of ADP-ribose are rather short-lived owing to the activity of PAR glycohydrolase (PARG), which cleaves the polymer eventually into single ADP-ribose units (53). Numerous splice variants are expressed from a single PARG gene (33,34), of which only the full-length protein is targeted to the nucleus due to a strong nuclear localization signal in exon I (34). Indeed, PARG activity is predominantly detected in cytoplasmic fractions (34,39,55), which is in apparent contradiction to the nuclear localization of PARP1, the clearly predominant PARP isoform (4,43,47).…”

“…A detailed analysis of PARG transcripts revealed novel splice variants. According to immunocytochemical studies, at least one isoform was clearly associated with mitochondria, although the suborganellar localization, and consequently its function, has remained unknown (33). A recent report suggested the dissociation of a PARG isoform from mitochondria following PARP1 activation in neuronal cells (44).…”