The monomer/dimer switch modulates the activity of plant adenosine kinase

Abstract Highlight The switch from active monomers to inactive dimers in plant ADKs impacts overall enzyme activity and represents a novel negative feedback-loop mechanism to maintain steady levels of adenosine and AMP. Adenosine undergoes ATP-dependent phosphorylation catalyzed by adenosine kinase (ADK). In plants, ADK also phosphorylates cytokinin ribosides, transport forms of the hormone. Here, we investigated the substrate preferences, oligomeric states and structures of ADKs from moss (Physcomitrella patens) and maize (Zea mays) alongside metabolomic and phenotypic analyses. We showed that dexamethasone-inducible ZmADK overexpressor lines in Arabidopsis can benefit from a higher number of lateral roots and larger root areas under nitrogen starvation. We discovered that maize and moss enzymes can form dimers upon increasing protein concentration, setting them apart from the monomeric human and protozoal ADKs. Structural and kinetic analyses revealed a catalytically inactive unique dimer. Within the dimer, both active sites are mutually blocked. The activity of moss ADKs, exhibiting a higher propensity to dimerize, was tenfold lower compared to maize ADKs. Two monomeric structures in a ternary complex highlight the characteristic transition from an open to a closed state upon substrate binding. This suggests that the oligomeric state switch can modulate the activity of moss ADKs and likely other plant ADKs. Moreover, dimer association represents a novel negative feedback mechanism, helping to maintain steady levels of adenosine and AMP. Competing Interest Statement The authors have declared no competing interest. Footnotes Abbreviations: ADK, adenosine kinase; Ado, adenosine; AMP, adenosine monophosphate; AP5A, diadenosine pentaphosphate; APT, adenine phosphoribosyltransferase; BAPR, 6-benzylaminopurine riboside; cZ, cis-zeatin; cZR, cis-zeatin riboside; cZRMP, cis-zeatin riboside 5’-monophosphate; DLS, dynamic light scattering; iP, N6-isopentenyl adenine; iPR, N6-isopentenyl adenosine; iPRMP, N6-isopentenyl adenosine 5’-monophosphate; LDH, lactate dehydrogenase; LOG, LONELY GUY phosphoribohydrolase; MST, microscale thermophoresis; NRH, nucleoside N-ribohydrolase; PK, pyruvate kinase; PNP, purine nucleoside phosphorylase; PpADK, adenosine kinase from Physcomitrium patens; SAM, S-adenosyl-L-methionine; tZ, trans-zeatin; tZR, trans-zeatin riboside; WT, wild type, ZmADK, adenosine kinase from Zea mays