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Acta Crystallogr D Biol Crystallogr. 2007 Mar;63(Pt 3):295-301. Epub 2007 Feb 21.

His-tag impact on structure.

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Center for Biophysical Sciences and Engineering, University of Alabama at Birmingham, 251 CBSE, 1025 18th Street South, Birmingham, AL 35294-4400, USA.


Crystallographers are increasingly determining structures of protein constructs that include His tags. Many have taken for granted that these tags have little effect on the native structure. This paper surveys and compares crystal structures with and without His tags. It is observed that actual refined tag residues fitted into density occur in less that 10% of the tagged sequences. However, higher resolution crystals are observed when this occurs. It is shown that these purification tags generally have no significant effect on the structure of the native protein. Resolution and R factors are not affected, but the overall B factors are slightly higher. Additional annotation in the PDB format to make tag definition explicit is suggested.

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