| Primary Identifier | IPR005511 | Type | Family |
| Short Name | SMP-30 |
| description | This entry represents members of the SMP-30/CGR1 family which act as lactonases such as regucalcin. Regucalcin (RGN) is a gluconolactonase (), converting D-glucono-1,5-lactone to D-gluconate, but also hydrolyzes other carbohydrate lactones. This enzyme is required for the penultimate step in vitamin C biosynthesis. From its crystal structure, regucalcin has a six-bladed β-propeller fold, and binds a single metal ion (either Ca2+or Zn2+)) []. Homologues with similar catalytic activity have been isolated and characterized from bacteria []. There are other bacterial homologues. L-arabinolactonase (), from Azospirillum brasilense, converts L-arabino-gamma-lactone to L-arabonate, allowing the bacterium utilize L-arabinose as a sole carbon source []; lactonase drp35 from Staphylococcusspecies acts as a lactonase on dihydrocoumarin or 2-coumaranone [].A homologue from the squid Loligo vulgarican act as a diisopropyl-fluorophosphatase (), but its physiological substrate is unknown [].Regucalcin, also known as senesence marker protein-30 (SMP30), was discovered in 1978 as a Ca2+binding protein that does not contain EF-hand motifs, suggesting a novel class of Ca2+binding protein. It is primarily localised to the liver and kidney cortex of animals. Expression of its mRNA in the liver and renal cortex of rats is stimulated by an increase in cellular Ca2+levels [, ]. Regucalin, as a regulatory protein of Ca2+, has a pivotal role in thecontrol of many cell functions. The protein has a reversible effect on Ca2+-induced activation and inhibition of many enzymes in both the liver and renal cortex cells []. It has also been shown to inhibit various protein kinases (including Ca2+/calmodulin-dependent protein kinase [], protein kinase C []and tyrosine kinase) and protein phosphatases, indicating a regulatory role in signal transduction within the cell. In addition, regucalcin regulates intracellular Ca2+homeostasis by enhancing Ca2+-pumping activity in the plasma membrane through activation of the pump enzymes []. Moreover, it can inhibit RNA synthesis in the nuclei of normal and regenerating rat livers in vitro [].Hydropathy profiles indicate hydrophobic domains in both N- and C-terminal regions of the regucalcin molecule; the protein also exhibits hydrophilic characteristics. Human and rodent regucalcins share 89% sequence identity, the high degree of conservation between species suggesting that the complete structure is required for physiological function. SMP30 sequences also share a high level of similarity with proteins from awide taxonomic range: these include fly anterior fat body proteins; fireflyluciferin regenerating enzyme; putative calcium binding transcriptionalregulatory proteins from Rhizobium meliloti and Streptomyces coelicolor;gluconolactonase from Brucella melitensis; cell growth protein CGR1 fromCandida albicans; and homologues from Thermoplasma acidophilum, Thermoplasmavolcanium, Sulfolobus tokodaii, Sulfolobus solfataricus, Bacillus subtilisand Rhizobium loti. As such, a number of lactonases are included in this family. |
