V67L Mutation Fills an Internal Cavity To Stabilize RecA Mtu Intein

Inteins mediate protein splicing, which has found extensive applications in protein science and biotechnology. In the Mycobacterium tuberculosis RecA mini-mini intein (Delta Delta Ihh), a single valine to leucine substitution at position 67 (V67L) dramatically increases intein stability and activity. However, crystal structures show that the V67L mutation causes minimal structural rearrangements, with a root-mean-square deviation of 0.2 angstrom between Delta Delta Ihh-V67 and Delta Delta Ihh-L67. Thus, the structural mechanisms for V67L stabilization and activation remain poorly understood. In this study, we used intrinsic tryptophan fluorescence, high-pressure nuclear magnetic resonance (NMR), and molecular dynamics (MD) simulations to probe the structural basis of V67L stabilization of the intein fold. Guanidine hydrochloride denaturation monitored by fluorescence yielded free energy changes (Delta Gf degrees) of -4.4 and -6.9 kcal mol(-1) for vIhh-V67 and Delta Delta Ihh-L67, respectively. High-pressure NMR showed that Delta Delta Ihh-L67 is more resistant to pressure-induced unfolding than Delta Delta Ihh-V67 is. The change in the volume of folding (Delta V-f) was significantly larger for V67 (71 +/- 2 mL mol(-1)) than for L67 (58 +/- 3 mL mol(-1)) inteins. The measured difference in Delta V-f (13 +/- 3 mL mol(-1)) roughly corresponds to the volume of the additional methylene group for Leu, supporting the notion that the V67L mutation fills a nearby cavity to enhance intein stability. In addition, we performed MD simulations to show that V67L decreases side chain dynamics and conformational entropy at the active site. It is plausible that changes in cavities in V67L can also mediate allosteric effects to change active site dynamics and enhance intein activity.