HOMEODOMAIN OF YEAST REPRESSOR ALPHA-2 CONTAINS A NUCLEAR-LOCALIZATION SIGNAL

The yeast repressor α2 is shown, by analysis of deletion-bearing α2-β-galactosidase hybrid proteins, to have two structurally distinct nuclear localization signals. The cellular location of hybrid proteins was determined by indirect immunofluorescence and optical sectioning of whole fixed yeast cells. The two nuclear localization signals are far apart in the α2 primary structure and do not have any sequence homology. One signal is, as reported previously, within the amino-terminal 13 amino acids of α2. Deletion of only this amino-terminal signal has no evident effect on nuclear localization. The second signal is in a central portion of α2, within the α2 homeodomain. Since this signal is within the amino terminus of the α2 homeodomain, the homeodomain mediates nuclear localization in addition to, and independently of, DNA binding. Deletion of only this second signal results in inefficient localization and accumulation of mutant protein at discrete sites on the nuclear envelope assumed to be nuclear pores. We propose that the two signals in α2 are functionally distinct and act at different steps in a localization pathway.