title: Serine protease activity of calnuc: regulation by Zn2+ and G proteins.
abstract: The functions of calnuc, a novel Ca(2+)-binding protein with multiple structural domains and diverse interacting partners, are yet unknown. We demonstrate unknown facets of calnuc, which is a serine protease in which Ser-378 of GXSXG motif, Asp-328 of DTG motif, and His-339 form the "catalytic triad," locating the enzyme active site in the C-terminal region. Analogous to the active site of Zn(2+) carboxypeptidases, calnuc has two high affinity (K(d) ∼ 20 nm), well conserved Zn(2+)-binding sites near its N terminus, although it is inactive as a peptidase. Zn(2+) binding allosterically and negatively regulates the serine protease activity of calnuc, inhibition being caused by an "open to close" change in its conformation not seen upon Ca(2+) binding. Most strikingly, interaction with G protein α subunit completely inhibits the enzymatic activity of calnuc. We thus illustrate that G proteins and Zn(2+) act as two "keys" that control enzymatic activity of calnuc, arresting it in "locked" state. Calnuc, therefore, exists dynamically in two different forms, (i) as a Ca(2+)-binding protein in Zn(2+)-bound form and (ii) as a protease in Zn(2+)-free form, commissioning it to perform multiple functions.
authors.affiliation: Department of Biotechnology, Indian Institute of Technology Madras, Chennai 600 036, India.
authors.forename: Madhavi
authors.initials: M
authors.lastname: Kanuru
authors.forename: Rajeev
authors.initials: R
authors.lastname: Raman
authors.forename: Gopala Krishna
authors.initials: GK
authors.lastname: Aradhyam
mesh_terms: D000494:Allosteric Regulation; D000495:Allosteric Site; D000595:Amino Acid Sequence; D000818:Animals; D001665:Binding Sites; D002118:Calcium / Q000378:metabolism; D002135:Calcium-Binding Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D020134:Catalytic Domain; D004268:DNA-Binding Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D004926:Escherichia coli / Q000235:genetics; D044385:GTP-Binding Protein alpha Subunits / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D015870:Gene Expression; D006801:Humans; D007700:Kinetics; D008969:Molecular Sequence Data; D009419:Nerve Tissue Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D000080866:Nucleobindins; D011485:Protein Binding; D011487:Protein Conformation; D011994:Recombinant Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D016415:Sequence Alignment; D017386:Sequence Homology, Amino Acid; D012697:Serine Endopeptidases / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D013816:Thermodynamics; D015032:Zinc / Q000737:chemistry / Q000378:metabolism*
chemical_list: D002135:Calcium-Binding Proteins; D004268:DNA-Binding Proteins; D044385:GTP-Binding Protein alpha Subunits; D009419:Nerve Tissue Proteins; D000080866:Nucleobindins; D011994:Recombinant Proteins; D012697:Serine Endopeptidases; D015032:Zinc; D002118:Calcium