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Most of the comments you will get will be anecdotal in that people will report the successful results and do not take the time or effort to characterize the less successful results. This often occurs because the tagged portion of the protein is most often disordered, even in the best crystals. Thus, other than saying "tagging on this end works, but tagging on that end doesn't," there is little more you can say. Each case will be different, and it is almost impossible to arrive at any generalized conclusion. We prefer C-terminal tagged proteins for a number of reasons, but if an N-terminally tagged protein crystallizes well, so be it. Of the dozens of N- and C-tagged protein structures we have solved in my lab and with collaborators, I have only seen one case of an ordered His-tag: the His residues had coordinated Cd ions, which proved essential for getting good crystals. However, beyond that there was not much more to say. For your protein and the resulting crystals, an N-terminally tagged protein crystallized well. Whether you can draw any more conclusions from these results depends on characterizing crystals of both N- and C-tagged proteins. Just assuming that the C-tagged protein is trying to crystallize in the same or related crystal form as the N-tagged protein is an unwarranted assumption without experimental evidence to back it up. That is why most groups just run with the winner. Cheers, Michael **************************************************************** R. Michael Garavito, Ph.D. Professor of Biochemistry & Molecular Biology 603 Wilson Rd., Rm. 513 Michigan State University East Lansing, MI 48824-1319 Office: (517) 355-9724 Lab: (517) 353-9125 FAX: (517) 353-9334 Email: [log in to unmask] **************************************************************** On Jun 26, 2012, at 9:06 PM, weliu wrote: > Dear all, > > We crystallized a protein and found that crystal quality greatly depended on the location of His-tag. When a His-tag was added at the C-terminus, only crystalline precipitate or spherical quasi crystals were grown. However, when the His-tag was moved to the N-terminus, single crystals were grown under a number of conditions, and the best one diffracted to 1.7 angstrom after optimization. I was wondering if there were published reports describing similar cases. > > Thank you in advance > > Wei Liu