IRBIT Promotes Allosteric Inhibition of Ribonucleotide Reductase

Some chemotherapeutic agents in clinical use are inhibitors of ribonucleotide reductase (RNR), which supplies the building blocks necessary for DNA synthesis and repair by converting ribonucleotide diphosphates to deoxynucleotide diphosphates, immediate precursors of deoxynucleotide triphosphates (dNTP). However, given that most of these compounds are nucleoside analogs that bind other nucleotide-binding proteins in addition to RNR, off-target interactions may contribute to the toxicity of these compounds. Arnaoutov and Dasso found that inositol-1,4,5-trisphosphate receptor-binding protein (IRBIT; also known as adenosylhomocysteinase-like 1, or AHCYL1) directly interacts with the R1 subunit of RNR in a deoxyadenosine triphosphate (dATP)–dependent manner. dATP is an allosteric regulator of RNR that either inhibits RNR by binding R1 at its low-affinity activity site (A-site) or promotes RNR catalytic activity by binding R1 at its specificity site (S-site). The authors showed that IRBIT binding to RNR inhibited the dissociation of dATP from the A-site of R1, thus stabilizing RNR in its inactive state and significantly inhibiting RNR activity in vitro. This inhibitory potential of IRBIT was modulated by phosphorylation, as a nonphosphorylatable IRBIT mutant had a reduced capacity to stabilize the dATP–R1 interaction. Depletion of IRBIT in asynchronously growing HeLa cells led to imbalanced dNTP levels; the imbalance was particularly pronounced during mitosis, when IRBIT was found to bind R1 more strongly compared with G₁. Knockdown of IRBIT also led to interphase length variability and accelerated mitotic progression, but the nonphosphorylatable IRBIT mutant was unable to rescue this phenotype, suggesting that IRBIT controls cell-cycle progression and that IRBIT phosphorylation is necessary for this role. These findings raise the possibility that regulation of RNR by IRBIT controls genomic stability and ensures proper cell-cycle progression by maintaining balanced intracellular dNTP pools and provide a framework for the development of RNR-selective inhibitors that may be less toxic than RNR inhibitors currently in use.

Arnaoutov A, Dasso M. IRBIT is a novel regulator of ribonucleotide reductase in higher eukaryotes. Science 2014;345:1512–5.

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