The C-terminal domain of dimeric serine hydroxymethyltransferase plays a key role in stabilization of the quaternary structure and cooperative unfolding of protein: domain swapping studies with enzymes having high sequence identity.
The serine hydroxymethyltransferase from Bacillus subtilis (bsSHMT) and B. stearothermophilus (bstSHMT) are both homodimers and share approximately 77% sequence identity; however, they show very different thermal stabilities and unfolding pathways. For investigating the role of N- and C-terminal domains in stability and unfolding of dimeric SHMTs, we have swapped the structural domains between bs- and bstSHMT and generated the two novel chimeric proteins bsbstc and bstbsc, respectively. The chimeras had secondary structure, tyrosine, and pyridoxal-5'-phosphate microenvironment similar to that of the wild-type proteins. The chimeras showed enzymatic activity slightly higher than that of the wild-type proteins. Interestingly, the guanidium chloride (GdmCl)-induced unfolding showed that unlike the wild-type bsSHMT, which undergoes dissociation of native dimer into monomers at low guanidium chloride (GdmCl) concentration, resulting in a non-cooperative unfolding of enzyme, its chimera bsbstc, having the C-terminal domain of bstSHMT was resistant to low GdmCl concentration and showed a GdmCl-induced cooperative unfolding from native dimer to unfolded monomer. In contrast, the wild-type dimeric bstSHMT was resistant to low GdmCl concentration and showed a GdmCl-induced cooperative unfolding, whereas its chimera bstbsc, having the C- terminal domain of bsSHMT, showed dissociation of native dimer into monomer at low GdmCl concentration and a GdmCl-induced non-cooperative unfolding. These results clearly demonstrate that the C-terminal domain of dimeric SHMT plays a vital role in stabilization of the oligomeric structure of the native enzyme hence modulating its unfolding pathway.
Division of Molecular and Structural Biology, Central Drug Research Institute, Lucknow 226 001, India. bhakuniv@rediffmail.com
Anant Narayan
AN
Bhatt
M Yahiya
MY
Khan
Vinod
V
Bhakuni
D000595:Amino Acid Sequence / Q000235:genetics; D001412:Bacillus subtilis / Q000201:enzymology* / Q000235:genetics; D001426:Bacterial Proteins / Q000737:chemistry* / Q000235:genetics; D019281:Dimerization; D001411:Geobacillus stearothermophilus / Q000201:enzymology* / Q000235:genetics; D012696:Glycine Hydroxymethyltransferase / Q000737:chemistry* / Q000235:genetics; D019791:Guanidine / Q000737:chemistry; D008969:Molecular Sequence Data; D011489:Protein Denaturation / Q000235:genetics; D017510:Protein Folding; D017434:Protein Structure, Tertiary / Q000235:genetics; D017386:Sequence Homology, Amino Acid
D001426:Bacterial Proteins; D012696:Glycine Hydroxymethyltransferase; D019791:Guanidine