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Ion‐binding properties of Calnuc, Ca 2+ versus Mg 2+ – Calnuc adopts additional and unusual Ca 2+ ‐binding sites upon interaction with G‐protein
Author(s) -
Kanuru Madhavi,
Samuel Jebakumar J.,
Balivada Lavanya M.,
Aradhyam Gopala K.
Publication year - 2009
Publication title -
the febs journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.981
H-Index - 204
eISSN - 1742-4658
pISSN - 1742-464X
DOI - 10.1111/j.1742-4658.2009.06977.x
Subject(s) - isothermal titration calorimetry , heterotrimeric g protein , calcium binding protein , binding site , protein subunit , biochemistry , binding protein , golgi apparatus , chemistry , biophysics , biology , g protein , calcium , endoplasmic reticulum , signal transduction , organic chemistry , gene
Calnuc is a novel, highly modular, EF‐hand containing, Ca 2+ ‐binding, Golgi resident protein whose functions are not clear. Using amino acid sequences, we demonstrate that Calnuc is a highly conserved protein among various organisms, from Ciona intestinalis to humans. Maximum homology among all sequences is found in the region that binds to G‐proteins. In humans, it is known to be expressed in a variety of tissues, and it interacts with several important protein partners. Among other proteins, Calnuc is known to interact with heterotrimeric G‐proteins, specifically with the α‐subunit. Herein, we report the structural implications of Ca 2+ and Mg 2+ binding, and illustrate that Calnuc functions as a downstream effector for G‐protein α‐subunit. Our results show that Ca 2+ binds with an affinity of 7 μ m and causes structural changes. Although Mg 2+ binds to Calnuc with very weak affinity, the structural changes that it causes are further enhanced by Ca 2+ binding. Furthermore, isothermal titration calorimetry results show that Calnuc and the G‐protein bind with an affinity of 13 n m . We also predict a probable function for Calnuc, that of maintaining Ca 2+ homeostasis in the cell. Using Stains‐all and terbium as Ca 2+ mimic probes, we demonstrate that the Ca 2+ ‐binding ability of Calnuc is governed by the activity‐based conformational state of the G‐protein. We propose that Calnuc adopts structural sites similar to the ones seen in proteins such as annexins, c2 domains or chromogrannin A, and therefore binds more calcium ions upon binding to Giα. With the number of organelle‐targeted G‐protein‐coupled receptors increasing, intracellular communication mediated by G‐proteins could become a new paradigm. In this regard, we propose that Calnuc could be involved in the downstream signaling of G‐proteins.