Biochemical characterization of missense mutations in the Arf/Arl-family small GTPase Arl6 causing Bardet-Biedl syndrome

Bardet-Biedl syndrome (BBS) is a pleiotropically genetic disorder, whose etiology is linked to cilia. Mutations in the Arf/Arl-family GTPase Arl6 have been recently shown to be responsible for BBS type 3. Here we show that BBS mutations alter the guanine nucleotide-binding properties of Arl6. Specifically, substitution of 31st Threonine to Arginine selectively abrogates the GTP-binding ability of Arl6 without affecting GDP-binding/dissociating properties. Furthermore, all the BBS mutations in Arl6 result in low expression of the mutant proteins, which can be restored by the inhibition of the proteasome. These findings implicate that Arl6 mutants are destabilized and eliminated by the proteasome in cells, probably due to the altered nucleotide-binding properties.     

Binding of ATP to TK1-like enzymes is associated with a conformational change in the quaternary structure

Human thymidine kinase 1 (hTK1) and structurally related TKs from other organisms catalyze the initial phosphorylation step in the thymidine salvage pathway. Though ATP is known to be the preferred phosphoryl donor for TK1-like enzymes, its exact binding mode and effect on the oligomeric state has not been analyzed. Here we report the structures of hTK1 and of the Thermotoga maritima thymidine kinase (TmTK) in complex with the bisubstrate inhibitor TP4A. The TmTK-TP4A structure reveals that the adenosine moiety of ATP binds at the subunit interface of the homotetrameric enzyme and that the majority of the ATP-enzyme interactions occur between the phosphate groups and the P-loop. In the hTK1 structure the adenosine group of TP4A exhibited no electron density. This difference between hTK1 and TmTK is rationalized by a difference in the conformation of their quaternary structure. A more open conformation, as seen in the TmTK-TP4A complex structure, is required to provide space for the adenosine moiety. Our analysis supports the formation of an analogous open conformation in hTK1 upon ATP binding.     

Antagonism of ethanol-induced decrease in rat brain cGMP concentration by histidyl-proline diketopiperazine, a thyrotropin releasing hormone metabolite.

Intraperitoneal administration of thyrotropin releasing hormone (50 μmol/kg) produced an approximately 2-fold increase in rat brain cGMP concentration within 15 min. Histidyl-proline diketopiperazine, a metabolite of thyrotropin releasing hormone, produced a similar effect, but the response was faster and shorter-lasting. Intraperitoneal administration of ethanol (1.5 g/kg) decreased brain cGMP concentration approximately 50% within 10–15 min; thyrotropin releasing hormone or histidyl-proline diketopiperazine, injected 5 min after ethanol, antagonized the ethanol-induced decrease in cGMP. Antagonism of the ethanol-induced decrease in the cGMP level required 10 μmol/kg of thyrotropin releasing hormone but was observed with 5 μmol/kg of histidyl-proline diketopiperazine. These data suggest that the metabolic conversion of thyrotropin releasing hormone to histidylproline diketopiperazine might explain the previous observation that thyrotropin releasing hormone elevated the level of brain cGMP and antagonized the ethanolinduced decrease in brain cGMP concentration.