The Macro or A1pp domain is a module of about 180 amino acids which can bind ADP-ribose (an NAD metabolite) or related ligands. Binding to ADP-ribose could be either covalent or non-covalent [1]: in certain cases it is believed to bind non-covalently ...
The Macro or A1pp domain is a module of about 180 amino acids which can bind ADP-ribose (an NAD metabolite) or related ligands. Binding to ADP-ribose could be either covalent or non-covalent [1]: in certain cases it is believed to bind non-covalently [2]; while in other cases (such as Aprataxin) it appears to bind both non-covalently through a zinc finger motif, and covalently through a separate region of the protein [3]. This domain is found in a number of otherwise unrelated proteins. It is found at the C-terminus of the macro-H2A histone protein 4 and also in the non-structural proteins of several types of ssRNA viruses such as NSP3 from alpha-viruses and coronaviruses. This domain is also found on its own in a family of proteins from bacteria, archaebacteria and eukaryotes. The 3D structure of the SARS-CoV Macro domain has a mixed alpha/beta fold consisting of a central seven-stranded twisted mixed beta sheet sandwiched between two alpha helices on one face, and three on the other. The final alpha-helix, located on the edge of the central beta-sheet, forms the C terminus of the protein [4]. The crystal structure of AF1521 (a Macro domain-only protein from Archaeoglobus fulgidus) has also been reported and compared with other Macro domain containing proteins. Several Macro domain only proteins are shorter than AF1521, and appear to lack either the first strand of the beta-sheet or the C-terminal helix 5. Well conserved residues form a hydrophobic cleft and cluster around the AF1521-ADP-ribose binding site [5].
We have named this region the SPX domain after SYG1, Pho81 and XPR1. This 180 residue long domain is found at the amino terminus of a variety of proteins. In the yeast protein SYG1, the N-terminus directly binds to the G-protein beta subunit and inhi ...
We have named this region the SPX domain after SYG1, Pho81 and XPR1. This 180 residue long domain is found at the amino terminus of a variety of proteins. In the yeast protein SYG1, the N-terminus directly binds to the G-protein beta subunit and inhibits transduction of the mating pheromone signal [3]. Similarly, the N-terminus of the human XPR1 protein binds directly to the beta subunit of the G-protein heterotrimer leading to increased production of cAMP. These findings suggest that all the members of this family are involved in G-protein associated signal transduction. The N-termini of several proteins involved in the regulation of phosphate transport, including the putative phosphate level sensors PHO81 Swiss:P17442 from Saccharomyces cerevisiae and NUC-2 Swiss:Q01317 from Neurospora crassa, are also members of this family [4,5]. The SPX domain of S. cerevisiae low-affinity phosphate transporters Pho87 and Pho90 auto-regulates uptake and prevents efflux. This SPX dependent inhibition is mediated by the physical interaction with Spl2 [6] NUC-2 contains several ankyrin repeats Pfam:PF00023. Several members of this family are annotated as XPR1 proteins: the xenotropic and polytropic retrovirus receptor confers susceptibility to infection with murine xenotropic and polytropic leukaemia viruses (MLV) [1]. Infection by these retroviruses can inhibit XPR1-mediated cAMP signalling and result in cell toxicity and death [7]. The similarity between SYG1, phosphate regulators and XPR1 sequences has been previously noted, as has the additional similarity to several predicted proteins, of unknown function, from Drosophila melanogaster, Arabidopsis thaliana, Caenorhabditis elegans, Schizosaccharomyces pombe, and Saccharomyces cerevisiae, and many other diverse organisms [1,2,6,7]. In addition, given the similarities between XPR1 and SYG1 and phosphate regulatory proteins, it has been proposed that XPR1 might be involved in G-protein associated signal transduction and may itself function as a phosphate sensor [1].
