The effect of protein context on nuclear location signal function.

The effect of position and number and of another intracellular location signal on the activity of the nuclear location signal was investigated. A minimal signal was inserted into several sites within the polypeptide chain of pyruvate kinase. The results observed suggest that a nuclear location signal can function at a variety of positions within a protein but that in some locations its activity is masked. Multiple copies of a partially defective signal were integrated into pyruvate kinase. The data suggest that multiple signals can cooperate to enhance nuclear accumulation. A nuclear location signal failed to function when inserted into polyomavirus middle T but was active in an identical variant lacking the carboxy-terminal hydrophobic tail. We conclude that while a minimal nuclear location signal is sufficient for nuclear localization, its activity is crucially dependent on the protein context within which it is present.