Translocation of chromatin proteins to nucleoli—The influence of protein dynamics on post‐fixation localization

It is expected that the subnuclear localization of a protein in a fixed cell, detected by microscopy, reflects its position in the living cell. We demonstrate, however, that some dynamic nuclear proteins can change their localization upon fixation by either crosslinking or non‐crosslinking methods. We examined the subnuclear localization of the chromatin architectural protein HMGB1, linker histone H1, and core histone H2B in cells fixed by formaldehyde, glutaraldehyde, glyoxal, ethanol, or zinc salts. We demonstrate that some dynamic, weakly binding nuclear proteins, like HMGB1 and H1, may not only be unexpectedly lost from their original binding sites during the fixation process, but they can also diffuse through the nucleus and eventually bind in nucleoli. Such translocation to nucleoli does not occur in the case of core histone H2B, which is more stably bound to DNA and other histones. We suggest that the diminished binding of some dynamic proteins to DNA during fixation, and their subsequent translocation to nucleoli, is induced by changes of DNA structure, arising from interaction with a fixative. Detachment of dynamic proteins from chromatin can also be induced in cells already fixed by non‐crosslinking methods when DNA structure is distorted by intercalating molecules. The proteins translocated during fixation from chromatin to nucleoli bind there to RNA‐containing structures.