Structural characterization of the conformational change in calbindin-D28k upon calcium binding using differential surface modification analyzed by mass spectrometry.
Calbindin-D28k is a calcium binding protein with six EF hand domains. Calbindin-D28k 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-D28k is known to bind four calcium ions and upon calcium binding undergoes a conformational change. The structure of apo calbindin-D28k 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 an ordered state. The solution structure of disulfide-reduced holo-calbindin-D28k has been determined by NMR, while the structure of apo calbindin-D28k 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-D28k 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-D28k. 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.
Hobbs
Carey A
CA
Department of Molecular and Structural Biochemistry, North Carolina State University, Raleigh, North Carolina 27695, USA.
Deterding
Leesa J
LJ
Perera
Lalith
L
Bobay
Benjamin G
BG
Thompson
Richele J
RJ
Darden
Thomas A
TA
Cavanagh
John
J
Tomer
Kenneth B
KB
D000595:Amino Acid Sequence; D000818:Animals; D001059:Apoproteins / Q000737:chemistry; D064092:Calbindin 1; D064026:Calbindins; D003545:Cysteine / Q000737:chemistry; D004220:Disulfides / Q000737:chemistry; D020832:EF Hand Motifs; D006860:Hydrogen Bonding; D009682:Magnetic Resonance Spectroscopy; D008969:Molecular Sequence Data; D011485:Protein Binding; D011487:Protein Conformation; D017434:Protein Structure, Tertiary; D051381:Rats; D064030:S100 Calcium Binding Protein G / Q000737:chemistry* / Q000378:metabolism*; D021241:Spectrometry, Mass, Electrospray Ionization; D053719:Tandem Mass Spectrometry
D001059:Apoproteins; C576500:Calb1 protein, rat; D064092:Calbindin 1; D064026:Calbindins; D004220:Disulfides; D064030:S100 Calcium Binding Protein G; D003545:Cysteine