Construction of hybrid enzymes between the glyceraldehyde-3-phosphate dehydrogenases from the mesophilic Methanobacterium bryantii and the thermophilic Methanothermus fervidus by recombinant DNA techniques revealed that a short C-terminal fragment of the Mt. fervidus enzyme contributes largely to its thermostability. This C-terminal region appears to be homologous to the alpha 3-helix of eubacterial and eukaryotic glyceraldehyde-3-phosphate dehydrogenases which is involved in the contacts between the two domains of the enzyme subunit. Site-directed mutagenesis experiments indicate that hydrophobic interactions play an important role in these contacts.