overnight at 4 C. MBP-MutS-H6 was eluted using 20 ml buffer B + 10 mM maltose. Eluate was applied to $4 ml of Ni-NTA resin pre-equilibrated in buffer B. Bound MBP-MutS-H6 was washed four times using 20 ml buffer B + 25 mM imidazole. Bound MBP-MutS-H6 was eluted using buffer B + 1 M imidazole. Eluate was concentrated via ultrafiltration using Amicon Ultra 5 kDa MWCO at 4 C. Concentrated sample was dialyzed extensively against 2· storage buffer (100 mM Tris-HCl, pH 7.5, 200 mM NaCl, 0.2 mM EDTA and 0.2 mM DTT) using a Slide-A-Lyzer 10 kDa MWCO cassette at 4 C. Protein concentration was determined using A 280 and a calculated extinction coefficient of 119 070 M À1 cm À1 . Dialyzed sample was diluted using an equal volume of glycerol and stored at À20 