We have also initiated a structural study of a calcium binding protein, CALNUC. This protein in the calcium loaded state binds Galpha (Ga) in the Golgi. It is believed that CALNUC is regulated through its interaction with Galpha to modulate calcium concentration in the Golgi apparatus. CALNUC does not seem to effect the GTP hydrolysis in Galpha. Therefore we hypothesize that there are several different modes of binding to the Galpha. These different modes govern a subset of different functions that the Galpha would undertake to respond to a certain stimulus. We have constructed the CALNUC plasmid which encompasses the two EF hands. We now have the structure of the calcium binding domain of CALNUC. it posseses a typical calcium binding loop. We are characterizing its calcium binding and try to correlate binding affinity to its binding loop structure. The backbone dynamics of this protein has been measured and we're in the process of correlating that to function of this protein, specifically its Ga interaction. We hope to be able to deduce from the structure of CALNUC its specific function. So far from our calcium binding experiments we believe that its function is to buffer calcium, due to the lower calcium afinity relative to other calcium binding proteins that are associated with signaling. Interestingly CALNUC does interact with Ga. We are trying to express and purify Gai to study its specific interaction with CALNUC. We succesfully solved the structure of CALNUC. We showed that the protein does bind 2 calciums. We also determined that both bonding sites have similar binding affinity. The protein undergoes an unfolding event when the calciums are removed. This is unique for calcium binding protein family and we hypothesize that this is correlated to the function of the protein as calcium signaling as well as buffering protein. We recently determined the affinity of CALNUC towards the C-terminal helix of Gai3. We employed polarization anisotropy. The dissociation constant is quite weak which is in agreement with what has been observed in cell competition assays. We are now trying to determine the affinity towards the full length Gai3, with the goal of studying structural determinants in the complex of these two proteins that define their role in signal regulation. So far we have shown in vitro that the binding of CALNUC to Gai3 if it is true must be very weak. We are currently trying to characterize possible partners that might regulate this interaction. In order to be able to study larger complexes by NMR we developed and adapted new technology in the laboratory. We adopted new NMR experiments to study larger proteins as well as labeling procedures (deuteration) that was not previously available for our group. In addition we showed by careful experiments and controls that one can use scalar coupling in addition to chemical shift to map out molecular interactions. This enables one to probe allosteric process that was previously difficult to distinguish from chemical shift information alone. At the same time we also showed that thru the use of paramagnetic spin label one can probe weak and transient interaction. We illustrated a protocol where one can determine a bound conformation of an amino acid binding protein, glutamine binding protein (GlnBP), from a known free or apo conformation using paramagnetic label alone. We further showed in the case of glutamine free form of GlnBP, using extensive paramagnetic relaxation enhancement data, does not sample the close conformation in solution. This is in contrast to Maltose binding protein that seems to transiently sample its close conformation, with 5% population, in solution. Based on what we learned here we can extend this protocols to look at various weak interactions in proteins involved in cell signaling cascades. In parallel we also observed a unique change of scalar coupling due to the presence of paramagnetic label. We believe that one contribution is due to the polarization of the spin orbital due to the electron field at the nucleus. In addition a contribution thru relaxation mechanism due to the interference between the electron dipole and nuclear dipole can not be ruled out. This is currently an on going project to test whether one can take advantage of this for structural information.
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