But, one thing we have found is that other proteins are often
co-purified--or sometimes purified instead of the desired his-tag fusion
protein--these usually turn out to be histidine-rich proteins, but they
are not the his-tag protein of choice.
Also, some proteins have not been obtained in very high yield--have you
check with amino acid composition to determine how much protein is really
in the sample?
Deb McMillen
Institute of Molecular Biology
University of Oregon
Eugene OR