Calnuc is a novel, highly modular, EF‐hand containing, Ca2+‐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 Ca2+ and Mg2+ binding, and illustrate that Calnuc functions as a downstream effector for G‐protein α‐subunit. Our results show that Ca2+ binds with an affinity of 7 μm and causes structural changes. Although Mg2+ binds to Calnuc with very weak affinity, the structural changes that it causes are further enhanced by Ca2+ binding. Furthermore, isothermal titration calorimetry results show that Calnuc and the G‐protein bind with an affinity of 13 nm. We also predict a probable function for Calnuc, that of maintaining Ca2+ homeostasis in the cell. Using Stains‐all and terbium as Ca2+ mimic probes, we demonstrate that the Ca2+‐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.

Calnuc is a ubiquitously expressed protein of the EF-hand Ca-binding superfamily. Previous studies have implicated it in Ca-sensitive physiological processes, whereas details of its function and involvement in human diseases are lacking. Drawing upon the sequence homology of calnuc with calreticulin, we propose it functions as a molecular chaperone-like protein. In cells under thermal, chemical [urea and guanidinium chloride (GdmCl)], and acidic stress, calnuc exhibits properties similar to those of established chaperone-like proteins (GRP78, spectrin, and α-crystallin), effectively demonstrated by its ability to suppress aggregation of malate dehydrogenase (MDH), alcohol dehydrogenase, and catalase. Calnuc aids in refolding of MDH with retention of 80% of its enzymatic activity. In HEK293 cells subjected to heat shock, calnuc chaperones luciferase, protecting its activity. Our in vitro and cell culture results establish the ability of calnuc to inhibit fibrillation of insulin and lysozyme and validate its neuroprotective role in cells treated with amyloid fibrils. Calnuc also rescues cells from fibrillar toxicity (caused by misfolded or aggregated proteins), providing a plausible explanation for the previous observation of its low level of expression in brains affected by Alzheimer's disease. We propose that calnuc is possibly involved in controlling protein unfolding diseases, such as Alzheimer's disease (AD), Parkinson's disease (PD), prion disease, and type II diabetes.

SummaryCalnuc is a recently discovered multidomain protein with EF-hand calcium binding sites. Several studies have reported various interacting partners for calnuc and, therefore, also different sites of localization in the cell. It interacts with important molecules such as DNA, G protein, COX, and amyloid precursor protein among others in addition to being involved in stress response and trafficking. The immense possibilities (of various functions this protein might be involved in) implicate great future in medicine and physiology. Preliminary studies also implicate the possibility of calnuc being involved in some of the human diseases. These initial observations imply the functions that this protein might be involved in. This review emphasizes the importance of further research on this protein. IUBMBIUBMB Life, 62(6): [436][437][438][439][440][441][442][443][444][445][446] 2010

Background: Calnuc is a multidomain Ca 2ϩ -binding protein with many interacting partners but whose function is still elusive. Results: Calnuc is a serine protease with its active site catalytic triad present in the C-terminal domain. Conclusion:The serine protease activity of calnuc is allosterically regulated by Zn 2ϩ -binding and its interaction with G protein ␣ subunit. Significance: Novel proteolytic function of calnuc will have vital implications in its physiological role.