Homo-and heterodimerization of the opioid receptors with functional consequences were reported previously. However, the exact nature of these putative dimers has not been identified. In current studies, the nature of the heterodimers was investigated by producing the phenotypes of the 1:1 heterodimers formed between the constitutively expressed -opioid receptor (MOR) and the ponasterone A-induced expression of ␦-opioid receptor (DOR) in EcR293 cells. By examining the trafficking of the cell surface-located MOR and DOR, we determined that these two receptors endocytosed independently. Using cell surface expression-deficient mutants of MOR and DOR, we observed that the corresponding wild types of these receptors could not rescue the cell surface expression of the mutants, whereas the antagonist naloxone could. Furthermore, studies with constitutive or agonist-induced receptor internalization also indicated that MOR and DOR endocytosed independently and could not "drag in" the corresponding wild types or endocytosis-deficient mutants. Additionally, the heterodimer phenotypes could be eliminated by the pretreatment of the EcR293 cells with pertussis toxin and could be modulated by the deletion of the RRITR sequence in the third intracellular loop that is involved in the receptor-G protein interaction and activation. These data suggest that MOR and DOR heterodimerize only at the cell surface and that the oligomers of opioid receptors and heterotrimeric G protein are the bases for the observed MOR-DOR heterodimer phenotypes.The ability of G protein-coupled receptors (GPCRs) 1 to homoor heterodimerize has implications in the functions of the receptors. Dimerization of the receptors has been reported for the class A GPCRs such as the adenosine (1), adrenergic (2-5), angiotensin (6), dopamine (7,8), muscarinic (9), vasopressin (2, 10), and opioid (11-15) receptors and the class C GPCRs such as the calcium-sensing (16), the metaboropic glutamate receptors (17), and the ␥-amino-n-butyric acid type B (GABA B ) receptors (18 -20). The homo-and heterodimerization of these receptors have been demonstrated by co-immunoprecipitation experiments (11, 21) and subsequently by the fluorescence resonance energy transfer or bioluminescence resonance energy transfer techniques (3,12,23). The heterodimerization of the GPCRs was shown to be selective, with formation of heterodimers with some but not all subtypes of the receptors (13, 24, 25). Most importantly, there are functional differences between the monomers and the homo-and heterodimers of the GPCRs. The classic example is the inability of individual GABA B1 and GABA B2 subunit to form a functional receptor (18 -20). Alteration in the GPCR function or expression was also observed with the heterodimerization of 5HT1B and -1D (26), dopamine D1 and adenosine A1 (27), muscarinic M2 and M3 (28), or dopamine and somatostatin (29) receptors. Heterooligomerization of the GPCRs with other receptor types, such as the ionotropic GABA A receptor, has been observed, resulting in the alteration ...

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Endonuclease III is a bifunctional DNA glycosylase that removes a wide range of oxidized bases in DNA. Deinococcus radiodurans is an extreme radiationresistant and desiccation-resistant bacterium and possesses three genes encoding endonuclease III enzymes in its genome: DR2438 (EndoIII-1), DR0289 (EndoIII-2) and DR0982 (EndoIII-3). Here, EndoIII-1 and an N-terminally truncated form of EndoIII-3 (EndoIII-3Á76) have been expressed, purified and crystallized, and preliminary X-ray crystallographic analyses have been performed to 2.15 and 1.31 Å resolution, respectively. The EndoIII-1 crystals belonged to the monoclinic space group C2, with unit-cell parameters a = 181.38, b = 38.56, c = 37.09 Å , = 89.34 and one molecule per asymmetric unit. The EndoIII-3Á76 crystals also belonged to the monoclinic space group C2, but with unit-cell parameters a = 91.47, b = 40.53, c = 72.47 Å , = 102.53 and one molecule per asymmetric unit. The EndoIII-1 structure was determined by molecular replacement, while the truncated EndoIII-3Á76 structure was determined by single-wavelength anomalous dispersion phasing. Refinement of the structures is in progress.

DNA ligases, the enzymes responsible for joining breaks in the phosphodiester backbone of DNA during replication and repair, vary considerably in size and structure. The smallest members of this enzyme class carry out their functions with pared-down protein scaffolds comprising only the core catalytic domains. Here we use sequence similarity network analysis of minimal DNA ligases from all biological super kingdoms, to investigate their evolutionary origins, with a particular focus on bacterial variants. This revealed that bacterial Lig C sequences cluster more closely with Eukaryote and Archaeal ligases, while bacterial Lig E sequences cluster most closely with viral sequences. Further refinement of the latter group delineates a cohesive cluster of canonical Lig E sequences that possess a leader peptide, an exclusively bacteriophage group of T7 DNA ligase homologs and a group with high similarity to the Chlorella virus DNA ligase which includes both bacterial and viral enzymes. The structure and function of the bacterially-encoded Chlorella virus homologs were further investigated by recombinantly producing and characterizing, the ATP-dependent DNA ligase from Burkholderia pseudomallei as well as determining its crystal structure in complex with DNA. This revealed that the enzyme has similar activity characteristics to other ATP-dependent DNA ligases, and significant structural similarity to the eukaryotic virus Chlorella virus including the positioning and DNA contacts of the binding latch region. Analysis of the genomic context of the B. pseudomallei ATP-dependent DNA ligase indicates it is part of a lysogenic bacteriophage present in the B. pseudomallei chromosome representing one likely entry point for the horizontal acquisition of ATP-dependent DNA ligases by bacteria.