N 6 -threonyl-carbamoylation of adenosine 37 of ANN-type tRNAs (t 6 A) is a universal modification essential for translational accuracy and efficiency. The t 6 A pathway uses two sequentially acting enzymes, YRDC and OSGEP, the latter being a subunit of the multiprotein KEOPS complex. We recently identified mutations in genes encoding four out of the five KEOPS subunits in children with Galloway-Mowat syndrome (GAMOS), a clinically heterogeneous autosomal recessive disease characterized by early-onset steroid-resistant nephrotic syndrome and microcephaly. Here we show that mutations in YRDC cause an extremely severe form of GAMOS whereas mutations in GON7 , encoding the fifth KEOPS subunit, lead to a milder form of the disease. The crystal structure of the GON7/LAGE3/OSGEP subcomplex shows that the intrinsically disordered GON7 protein becomes partially structured upon binding to LAGE3. The structure and cellular characterization of GON7 suggest its involvement in the cellular stability and quaternary arrangement of the KEOPS complex.

The universal N6-threonylcarbamoyladenosine (t6A) modification at position A37 of ANN-decoding tRNAs is essential for translational fidelity. In bacteria the TsaC enzyme first synthesizes an l-threonylcarbamoyladenylate (TC-AMP) intermediate. In cooperation with TsaB and TsaE, TsaD then transfers the l-threonylcarbamoyl-moiety from TC-AMP onto tRNA. We determined the crystal structure of the TsaB–TsaE–TsaD (TsaBDE) complex of Thermotoga maritima in presence of a non-hydrolysable AMPCPP. TsaE is positioned at the entrance of the active site pocket of TsaD, contacting both the TsaB and TsaD subunits and prohibiting simultaneous tRNA binding. AMPCPP occupies the ATP binding site of TsaE and is sandwiched between TsaE and TsaD. Unexpectedly, the binding of TsaE partially denatures the active site of TsaD causing loss of its essential metal binding sites. TsaE interferes in a pre- or post-catalytic step and its binding to TsaBD is regulated by ATP hydrolysis. This novel binding mode and activation mechanism of TsaE offers good opportunities for antimicrobial drug development.

The tRNA modification N6-threonylcarbamoyladenosine (t6A) is universally conserved in all organisms. In bacteria, the biosynthesis of t6A requires four proteins (TsaBCDE) that catalyze the formation of t6A via the unstable intermediate l-threonylcarbamoyl-adenylate (TC-AMP). While the formation and stability of this intermediate has been studied in detail, the mechanism of its transfer to A37 in tRNA is poorly understood. To investigate this step, the structure of the TsaBD heterodimer from Escherichia coli has been solved bound to a stable phosphonate isosteric mimic of TC-AMP. The phosphonate inhibits t6A synthesis in vitro with an IC50 value of 1.3 μM in the presence of millimolar ATP and L-threonine. The inhibitor binds to TsaBD by coordination to the active site Zn atom via an oxygen atom from both the phosphonate and the carboxylate moieties. The bound conformation of the inhibitor suggests that the catalysis exploits a putative oxyanion hole created by a conserved active site loop of TsaD and that the metal essentially serves as a binding scaffold for the intermediate. The phosphonate bound crystal structure should be useful for the rational design of potent, drug-like small molecule inhibitors as mechanistic probes or potentially novel antibiotics.

In Archaea and Eukaryotes, the synthesis of a universal tRNA modification, N6-threonyl-carbamoyl adenosine (t6A), is catalyzed by the KEOPS complex composed of Kae1, Bud32, Cgi121, and Pcc1. A fifth subunit, Gon7, is found only in Fungi and Metazoa. Here, we identify and characterize a fifth KEOPS subunit in Archaea. This protein, dubbed Pcc2, is a paralog of Pcc1 and is widely conserved in Archaea. Pcc1 and Pcc2 form a heterodimer in solution, and show modest sequence conservation but very high structural similarity. The five-subunit archaeal KEOPS does not form dimers but retains robust tRNA binding and t6A synthetic activity. Pcc2 can substitute for Pcc1 but the resulting KEOPS complex is inactive, suggesting a distinct function for the two paralogs. Comparative sequence and structure analyses point to a possible evolutionary link between archaeal Pcc2 and eukaryotic Gon7. Our work indicates that Pcc2 regulates the oligomeric state of the KEOPS complex, a feature that seems to be conserved from Archaea to Eukaryotes.

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In Archaea and Eukaryotes, the synthesis of a universal tRNA modification, t6A, is catalyzed by the KEOPS complex composed of Kae1, Bud32, Cgi121 and Pcc1. A fifth subunit, Gon7, is found only in Fungi and Metazoa. Mutations in all five genes encoding human KEOPS subunits leads to Galloway-Mowat syndrome, a severe genetic disease causing childhood lethality. Here, we describe the discovery and biochemical characterization of the archaeal fifth KEOPS subunit. This protein, dubbed Pcc2, is a paralog of Pcc1 and is widely conserved in Archaea. Pcc1 and Pcc2 form a heterodimer in solution, show modest sequence conservation but very high structural similarity. The 5-subunit KEOPS lost its capacity to form dimers but its tRNA binding and t6A synthetic activity remained robust. Pcc2 can substitute Pcc1 but the resulting KEOPS complex is inactive suggesting a distinct function for the two paralogs. Comparative sequence and structure analyses point to a possible evolutionary link between archaeal Pcc2 and eukaryotic Gon7 proteins. Our work thus reveals that Pcc2 has evolved to regulate the oligomeric state of KEOPS complex thus adding another layer of complexity to the biosynthesis of t6A that seems to be conserved from Archaea to Eukaryotes.

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