How to purify unstable protein? - (Dec/30/2011 )
the mutant eluting earlier than the wild type means that what you are eluting is greater in molecular weight. since you see multiple bands in sds-page, it is most likely an aggregate.
have you eluted the mutant and wt from nta without removing the his tag? if not then try it and see if you get more reasonable results.
if that gives you the expected result then you may want to see what effect protease treatment has on the proteins in solution.