Thanks for your valuable suggestions, to be more precise this protein is biologically dimeric in nature and we are getting same result through PISA. But during stressed conditions this wt protein forms tetramers (As seen in native gel), further mutant protein also forms tetramers under normal conditions (i.e. in the absence of stress, like low pH), so can we assume that the stronger interactions between symmetry mates seen in mutant w.e.t. wt protein are the cause of formation of tetrameric population even in the absence of stress in mutant. Further since PISA is predicting it to be dimer can we predict the structure of higher order oligomerisation state based on symmetry analysis results as well as by doing some mutations at symmetry interface to prove that in solution also, these interactions persists and are responsible for oligomerisation of this protein. Also I want to add that mutations are not in the interaction interface.