Arabidopsis inositol polyphosphate kinases regulate COP9 signalosome deneddylase functions in phosphate-homeostasis

Abstract

AbstractTargeted protein degradation is essential for physiological development and adaptation to stress. Mammalian INOSITOL PENTAKISPHOSPHATE 2-KINASE (IP5K) and INOSITOL HEXAKISPHOSPHATE KINASE 1 (IP6K1) pair generates inositol polyphosphates (InsPs) to modulate association/dissociation equilibrium of Cullin RING Ubiquitin E3 ligases (CRLs) on the COP9 signalosome (CSN) platform. Deneddylase activity of the CSN5 subunit protects cullins from self-ubiquitination ensuring their functional continuity. In plants, similar regulations by InsP-kinases are not known. Here, we show conserved interactions of Arabidopsis thaliana INOSITOL PENTAKISPHOSPHATE 2-KINASE 1 (IPK1) and INOSITOL 1,3,4-TRISPHOSPHATE 5/6-KINASE 1 (ITPK1), counterparts of the above mammalian InsP-kinase pair, with selective CSN subunits. In ipk1 or itpk1 mutants, deneddylation deficiencies not only cause increased neddylated Cullin1 (CUL1Nedd8) pools more prone to degradation but also impair CSN5 entry/exit shuttles on the CSN holo-complex. Constitutive phosphate-starvation response (PSR), previously known for these mutants are suppressed by pharmacological inhibition of neddylation thus linking CSN-CRL functions to phosphate (Pi)-sensing. Similarly, in wild-type plants exposed to compounds that impair CSN5 deneddylase function affects its dynamics and mimic PSR properties of the mutants. We further show that under Pi-deprivation more CSN5 retentions on the CSN holo-complex and the resulting enhanced CUL1Nedd8 pools is essential for induction of downstream Pi-starvation inducible (PSI) genes. Overall, with our data we present InsP-kinase involvements in maintenance of Pi-homeostasis in plants via CRL-CSN(5) functional synergism.SignificanceNeddylation modifications on Culling-RING E3 ligases (CRLs) while essential for their role in proteostasis, also threaten their own stability. Selective inositol polyphosphates (InsPs) aid the constitutive photomorphogenesis 9 signalosome (CSN) functions in protecting, deneddylating, and facilitating CRL recycling. Here we demonstrate that plant mutants deficient in these InsPs have disturbed CSN subunit partitioning, are deficient in deneddylase activity, and hyperaccumulate neddylated cullins that lead to constitutive phosphate-starvation response (PSR). Inhibition of CSN functions/consequences mirror the InsP mutant properties indicating pivotal involvement of CSN in phosphate homeostasis. These raises promising possibilities of targeted intervention on CSN functions for nutritional benefit of plants.