Metal-binding properties of human centrin-2 determined by micro-electrospray ionization mass spectrometry and UV spectroscopy

We analyzed the metal-binding properties of human centrin-2 (HsCen-2) and followed the changes in HsCen-2 structure upon metal-binding using micro-electrospray ionization mass spectrometry (muESI-MS). Apo-HsCen-2 is mostly monomeric. The ESI spectra of HsCen-2 show two charge-state distributions, representing two conformations of the protein. HsCen-2 binds four moles calcium/mol protein: one mol of calcium with high affinity, one additional mol of calcium with lower affinity, and two moles of calcium at low affinity sites. HsCen-2 binds four moles of magnesium/mol protein. The conformation giving the lower charge-state HsCen-2 by ESI, binds calcium and magnesium more readily than does the higher charge-state HsCen-2. Both conformations of HsCen-2 bind calcium more readily than magnesium. Calcium was more effective in displacing magnesium bound to HsCen-2 than vice versa. Binding of a peptide from a known binding partner, the xeroderma pigmentosum complementation group protein C (XPC), to apo-HsCen-2, occurs in the presence or the absence of calcium. Near and far-UV CD spectra of HsCen-2 show little difference with addition of calcium or magnesium. Minor changes in secondary structure are noted. Melting curves derived from temperature dependence of molar ellipticity at 222 nm for HsCen-2 show that calcium increases protein stability whereas magnesium does not. Delta 25 HsCen-2 behaves similarly to HsCen-2. We conclude that HsCen-2 binds calcium and magnesium and that calcium modulates HsCen-2 structure and function by increasing its stability without undergoing significant changes in secondary or tertiary structure.