Pentapeptide nuclear localization signal in adenovirus E1a.

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Abstract
The adenovirus E1a gene products are nuclear proteins important in transcriptional control of viral functions during infection. By producing normal E1a proteins and derivatives of E1a in bacteria and microinjecting these proteins into cultured cells, we were able to examine their ability to localize to the nucleus. We showed that a short peptide sequence at the carboxyl terminus of E1a is necessary for the rapid (30-min) nuclear localization of that protein. Additionally, we showed that just the last five amino acids of E1a are sufficient to direct nuclear accumulation of a heterologous protein, Escherichia coli galactokinase, with the same kinetics as native E1a. The mechanism by which this pentamer mediates rapid nuclear localization was examined by testing the ability of a galactokinase derivative which has no signal pentamer to exit the nucleus, as well as to enter it. Because neither free entry nor exit was detected, the effect of the signal is unlikely to be through increased nuclear retention of freely diffusible proteins but rather by enhancement of entry into the nucleus.