AglM and VNG1048G, Two Haloarchaeal UDP-Glucose Dehydrogenases, Show Different Salt-Related Behaviors.

Haloferax volcanii AglM and Halobacterium salinarum VNG1048G are UDP-glucose dehydrogenases involved in N-glycosylation in each species. Despite sharing >60% sequence identity and the ability of VNG1048G to functionally replace AglM in vivo, these proteins behaved differently as salinity changed. Whereas AglM was active in 2-4 M NaCl, VNG1048G lost much of its activity when salinity dropped below 3 M NaCl. To understand the molecular basis of this phenomenon, each protein was examined by size exclusion chromatrography in 2 M NaCl. Whereas AglM appeared as a dodecamer, VNG1048G was essentially detected as a dodecamer and a dimer. The specific activity of the VNG1048G dodecamer was only a sixth of that of AglM, while the dimer was inactive. As such, not only was the oligomeric status of VNG1048G affected by lowered salinity, so was the behavior of the individual dodecamer subunits. Analyzing surface-exposed residues in homology models of the two UDP-glucose dehydrogenases revealed the more acidic and less basic VNG1048G surface, further explaining the greater salt-dependence of the Hbt. salinarum enzyme.