Glycine-rich loop encompassing active site at interface of hexameric M. tuberculosis Eis protein contributes to its structural stability and activity

RvEis is a crucial thermostable hexameric aminoglycoside acetyltransferase of Mycobacterium tuberculosis, overexpression of which confers Kanamycin resistance in clinical strains. The thermostability associated with hexameric RvEis is important for the enhanced intracellular survival of mycobacteria. However, the structural determinants responsible for its thermal stability remain unexplored. In this study, we have assessed the role of glycines of conserved glycine-rich motif (G123GIYG127) present at the oligomeric interface in the hydrophobic core of RvEis in sustenance of its structural stability, oligomerization and functional activity. Substitution of glycines to alanine (G123A/G127A) result in significant decrease in melting temperature (Tm), reduction in the oligomerization with concomitant increase in the monomeric form and higher susceptibility towards the denaturants like GdmCl and urea relative to wild type. G123A/G127A mutant displayed lower catalytic efficiency (kcat/Km) and is completely inactive at 60 °C. ANS binding assay and the complete dissociation of hexameric complex into monomers at lower concentration of urea in G123A/G127A relative to wtRvEis suggests that altered hydrophobic environment could be the reason for its instability. In sum, these results demonstrate the role of G123GIYG127 motif in structural stability and activity of RvEis.