Effect of guanine nucleotides on the conformation and stability of chloroplast elongation factor Tu.

The effect of guanine nucleotides and kirromycin on the conformation and stability of the chloroplast elongation factor Tu (EF-Tuchl) from Euglena gracilis has been investigated. Free EF-Tuchl is quite thermolabile but the protein is greatly stabilized by guanine nucleotides. The temperature dependence of the thermal inactivation of EF-Tuchl was used to calculate the amount of stabilization energy conferred by the guanine nucleotides. GDP increases the activation energy for the denaturation process by 77 kcal/mol while GTP increases the activation energy by 51 kcal/mol. The difference in heat stability of free EF-Tuchl and the EF-Tuchl.GDP complex was used to determine a dissociation constant of 1.3 x 10(-7) M at 37 degrees C. The temperature dependence of the dissociation constant allowed the calculation of a delta H degree obsd of -55 kcal/mol and a delta S degree obsd of -146 cal/(mol degree) for GDP binding to EF-Tuchl.EF-Tuchl was found to have a trypsin-sensitive region similar to that observed for Escherichia coli EF-Tu. This loop region was protected by GTP and kirromycin but not by GDP.