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作者 |
Hobbs, CA; Deterding, LJ; Perera, L; Bobay, BG; Thompson, RJ; Darden, TA; Cavanagh, J; Tomer, KB |
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[摘要]：Calbindin-D-28k is a calcium binding protein with six EF hand domains. Calbindin-D-28k is unique in that it functions as both a calcium buffer and a sensor protein. It is found in many tissues, including brain, pancreas, kidney, and intestine, playing important roles in each. Calbindin-D-28k is known to bind four calcium ions and upon calcium binding undergoes a conformational change. The structure of apo calbindin-D-28k is in an ordered state, transitioning into a disordered state as calcium is bound. Once fully loaded with four calcium ions, it again takes on ail ordered state. The solution structure of disulfide-reduced holo-calbindin-D-28k has been determined by NMR, while the structure of apo calbindin-D-28k has yet to be determined. Differential surface modification of lysine and histidine residues analyzed by mass spectrometry has been used in this study to identify, for the first time, the specific regions of calbindin-D-28k undergoing conformational changes between the holo and apo states. Using differential Surface modification in combination with mass spectrometry, EF hands 1 and 4 as well as the linkers before EF hand 1 and the linkers between EF hands 4 and 5 and EF hands 5 and 6 were identified as regions of conformational change between apo and holo calbindin-D-28k. Under the experimental conditions employed, EF hands 2 and 6, which are known not to bind calcium, were unaffected in either form. EF hand 2 is highly accessible; however, EF hand 6 was determined not to be surface accessible in either form. Previous research has identified a disulfide bond between cysteines 94 and 100 in the holo state. Until now, it was unknown whether this bond also exists in the apo form. Our data confirm the presence of the disulfide bond between cysteines 94 and 100 in the holo form and indicate that there is predominantly no disulfide bond between these residues in the apoprotein. |
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