An electron microscopic study of the archaeal feast/famine regulatory protein: 3. Labeling of the N-terminus using nickel

Histidine residues added to the N-terminus of a polypeptide (i.e. a His-tag) was used, for the first time to our knowledge, for electron labeling of the protein upon its electron spectroscopic imaging. Originally such a His-tag was developed by another group to purify modified proteins by taking advantage of their affinity to nickel. The feast/famine regulatory protein pot0434017 (FL11) was modified by adding six His residues to its N-terminus, so that each His pair would chelate a nickel ion. An electron microscope was operated at 200 KeV, and the electrons that lost the energy by ~875 eV upon interaction with the metal were selectively focused. The majority, 60–70%, of the spots detected in the electron micrographs were paired by distances shorter than 80 Å, and over 70% of them were paired by distances shorter than 40 Å. It is concluded that the protein molecules formed dimers, and the termini of most of the protein molecules were labeled with nickel by this method.