You have a 25kD histidine tagged bacterial protein (100% pure). You apply this pure protein to a Ni+2 agarose column. Then you apply a crude mouse cell extract to the column. After extensive washing, you apply an imidazole buffer. Analysis of the resultant elution on an SDS-PAGE gel indicates two proteins (25kD and 75kD) of equal coomassie blue staining. What is the simplest explanation for why the 25kD and the 75kD proteins copurified off the column?