microheterogeneity and tagged proteins

[Alan H. Schulman]

I would like to express a protein in E. coli for ultimate crystallization.  It 
would be convenient to express it as a fusion protein.  Of the several systems 
available (6xHis (InVitroGen), strep-tag (Biometra),clip (Promega) etc., does 
anyone have experience with the result in crstallization?  How cleanly can the 
tag be cleaved off, without generating heterogeneity from multiple cleavage 
positions?

Alan Schulman
University of Helsinki
fax +358 0 434 6046
email schulman at convex.csc.fi