Coordination Structures of Mg~(2+) and Ca~(2+) in Three Types of Tobacco Calmodulins in Solution: Fourier-Transform Infrared Spectroscopic Studies of Side-Chain COO~- Groups

摘要

Calmodulin (CaM) is a Ca~(2+)-binding protein that regulates a number of fundamental cellular activities. Nicotiana tabacum CaM (NtCaM) comprises 13 genes classified into three types, among which gene expression and target enzyme activation differ. We performed Fourier-transform infrared spectroscopy to compare the secondary and coordination structures of Mg~(2+) and Ca~(2+) among NtCaM1, NtCaM3, and NtCaM13 as representatives of the three types of NtCaMs. Data suggested that NtCaM13 has a different secondary structure due to the weak β-strand bands and the weak1661 cm21 band. Coordination structures of Mg~(2+) of NtCaM3 and NtCaM13 were similar but different from that of NtCaM1, while the Ca~(2+)-binding manner was similar among the three CaMs. The amplitude differenc of the band at 1554-1550 cm21 obtained by secondderivative spectra indicated that the intensity change of the band of NtCaM13 was smaller in response to [Ca~(2+)] increases under low [Ca~(2+)] conditions than were those of NtCaM1 and NtCaM3, while the intensity reached the same level under high [Ca~(2+)]. Therefore, NtCaM13 has a characteristic secondary structure and specific Mg~(2+)-binding manner and needs higher [Ca~(2+)] for bidentate Ca~(2+) coordination of 12th Glu in EF-hand motifs. The Ca~(2+)-binding mechanisms of the EF-hand motifs of the three CaMs are similar; however, the cationdependent conformational change in NtCaM13 is unique among the three NtCaMs.