Structure, binding interface and hydrophobic transitions of Ca2+-loaded calbindin-D-28K
Calbindin-D-28K is a Ca2+-binding protein, performing roles as both a calcium buffer and calcium sensor. The NMR solution structure of Ca2+-loaded calbindin-D-28K reveals a single, globular fold consisting of six distinct EF-hand subdomains, which coordinate Ca2+ in loops on EF1, EF3, EF4 and EF5. Target peptides from Ran-binding protein M and myo-inositol monophosphatase, along with a new target from procaspase-3, are shown to interact with the protein on a surface comprised of alpha 5 (EF3), alpha 8 (EF4) and the EF2-EF3 and EF4-EF5 loops. Fluorescence experiments reveal that calbindin-D28K adopts discrete hydrophobic states as it binds Ca2+. The structure, binding interface and hydrophobic characteristics of Ca2+- loaded calbindin-D-28K provide the first detailed insights into how this essential protein may function. This structure is one of the largest highresolution NMR structures and the largest monomeric EF-hand protein to be solved to date.