Eukaryotic and archaeal translation initiation factors 2, heterotrimers that consist of alpha-, beta-, and gamma-subunits, deliver methionylated initiator tRNA to a small ribosomal subunit in a manner that depends on GTP. To evaluate correlation of the function and association of the subunits, we used isothermal titration calorimetry to analyze the thermodynamics of the interactions between the alpha- and gamma-subunits in the presence or absence of a nonhydrolyzable GTP analog or GDP. The alpha-subunits bound to the gamma-subunit with large heat capacity change (DeltaC(p)) values. The DeltaH and DeltaC(p) values for the interaction between the alpha- and gamma-subunits varied in the presence of the GTP analog but not in the presence of GDP. These results suggest that the binding of both the alpha-subunit and GTP changes the conformation of the switch region of the gamma-subunit and increases the affinity of the gamma-subunit for tRNA.