InterPro domain: IPR005511

Regucalcin, also known as senesence marker protein-30 (SMP30), was discovered in 1978 as a Ca ²⁺ binding protein that does not contain EF-hand motifs, suggesting a novel class of Ca ²⁺ 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 Ca ²⁺ levels [ 1 , 2 ].

Regucalin, as a regulatory protein of Ca ²⁺ , has a pivotal role in thecontrol of many cell functions. The protein has a reversible effect on Ca ²⁺ -induced activation and inhibition of many enzymes in both the liver and renal cortex cells [ 3 ]. It has also been shown to inhibit various protein kinases (including Ca ²⁺ /calmodulin-dependent protein kinase [ 4 ], protein kinase C [ 5 ] and tyrosine kinase) and protein phosphatases, indicating a regulatory role in signal transduction within the cell. In addition, regucalcin regulates intracellular Ca ²⁺ homeostasis by enhancing Ca ²⁺ -pumping activity in the plasma membrane through activation of the pump enzymes [ 6 ]. Moreover, it can inhibit RNA synthesis in the nuclei of normal and regenerating rat livers in vitro [ 7 ].

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.

This entry represents members of the SMP-30/CGR1 family which act as lactonases such as regucalcin. Regucalcin (RGN) is a gluconolactonase ( 3.1.1.17 ), 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 beta-propeller fold, and binds a single metal ion (either Ca ²⁺ or Zn ²⁺ )) [ 8 ]. Homologues with similar catalytic activity have been isolated and characterized from bacteria [ 9 ]. There are other bacterial homologues. L-arabinolactonase ( 3.1.1.15 ), from Azospirillum brasilense , converts L-arabino-gamma-lactone to L-arabonate, allowing the bacterium utilize L-arabinose as a sole carbon source [ 10 ]; lactonase drp35 from Staphylococcus species acts as a lactonase on dihydrocoumarin or 2-coumaranone [ 11 ].

A homologue from the squid Loligo vulgari can act as a diisopropyl-fluorophosphatase ( 3.1.8.2 ), but its physiological substrate is unknown [ 12 ].

1. Molecular cloning and sequencing of the cDNA coding for a calcium-binding protein regucalcin from rat liver. FEBS Lett. 327, 251-5
2. Calcium administration stimulates the expression of calcium-binding protein regucalcin mRNA in rat liver. FEBS Lett. 305, 151-4
3. Reversible effect of calcium-binding protein regucalcin on the Ca(2+)-induced inhibition of deoxyuridine 5'-triphosphatase activity in rat liver cytosol. Mol. Cell. Biochem. 110, 25-9
4. Hepatic calcium-binding protein regucalcin decreases Ca2+/calmodulin-dependent protein kinase activity in rat liver cytosol. Chem. Pharm. Bull. 38, 2216-8
5. Inhibitory effect of calcium-binding protein regucalcin on protein kinase C activity in rat renal cortex cytosol. Biol. Pharm. Bull. 21, 315-8
6. Regulatory effect of regucalcin on (Ca(2+)-Mg2+)-ATPase in rat liver plasma membranes: comparison with the activation by Mn2+ and Co2+. Mol. Cell. Biochem. 124, 169-74
7. Inhibitory effect of calcium-binding protein regucalcin on ribonucleic acid synthesis in isolated rat liver nuclei. Mol. Cell. Biochem. 173, 169-75
8. Crystal structure of human senescence marker protein 30: insights linking structural, enzymatic, and physiological functions . Biochemistry 49, 3436-44
9. Isolation and characterization of the gene encoding gluconolactonase from Zymomonas mobilis. Biochim. Biophys. Acta 1171, 198-200
10. Identification and characterization of L-arabonate dehydratase, L-2-keto-3-deoxyarabonate dehydratase, and L-arabinolactonase involved in an alternative pathway of L-arabinose metabolism. Novel evolutionary insight into sugar metabolism. J. Biol. Chem. 281, 33521-36
11. Staphylococcal Drp35 is the functional counterpart of the eukaryotic PONs. FEMS Microbiol. Lett. 249, 185-90
12. Mutational and structural studies of the diisopropylfluorophosphatase from Loligo vulgaris shed new light on the catalytic mechanism of the enzyme. Biochemistry 44, 9022-33