For increased stability, particularly at higher ionic strengths, RB3SLD was mutated at four positions that are modeled in the structure, F20W (colored cyan), K85R (colored light green), F93W (colored yellow), and L116F (colored orange)  (numbering is in reference to stathmin). 

These mutations counteract to some extent the detrimental effect of salt on T2R stability. Three of them introduce bulkier and aromatic side chains prone to interact with hydrophobic patches of tubulin. In particular, Trp20 targets the same α tubulin pocket. The fourth mutation (K85R) was aimed at reinforcing the RB3SLD interaction with nearby tubulin acidic residues (Asp β414 and Glu β417).

4F61 (PDB)