We report here evidence for the mechanism of nuclear localization of X
PG nuclease in human cells, Several candidate nuclear localization sig
nal (NLS) peptides have been proposed for XPG protein. We have identif
ied XPG peptides containing functional NLS and a potential nuclear ret
ention signal (NRS) using in situ immune-fluorescence localization of
transiently expressed beta-galactosidase fusion proteins. Two XPG regi
ons with putative NLS [amino acid (AA) coordinates: NLS-B (AA 1057-107
4) and NLS-C (AA 1171-1185)] were each shown to independently localize
the beta-gal extensively (> 80%) to the nucleus of HeLa cells. The C-
terminus peptide containing NLS-C, all NLS conserved evolutionarily be
tween yeasts and humans, also directed sub-localization of beta-galact
osidase to intranuclear foci reminiscent of native XPG protein, as wel
l as to peri-nucleolar regions. Peptides in the putative XPG 'NLS doma
in' (AA similar to 1051-1185) apparently function in concert for nucle
ar localization and also for retention of XPG in nuclear matrix-associ
ated foci. Evidence presented elsewhere (Park et al., 1995) indicates
that the peptide containing NLS-C (AA 1146-1185) also regulates the dy
namic localization of XPG in the nucleus following UV-irradiation.