Homeodomain of yeast repressor alpha 2 contains a nuclear localization signal.

The yeast repressor alpha 2 is shown, by analysis of deletion-bearing alpha 2-beta-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 alpha 2 primary structure and do not have any sequence homology. One signal is, as reported previously, within the aminoterminal 13 amino acids of alpha 2. Deletion of only this aminoterminal signal has no evident effect on nuclear localization. The second signal is in a central portion of alpha 2, within the alpha 2 homeodomain. Since this signal is within the amino terminus of the alpha 2 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 alpha 2 are functionally distinct and act at different steps in a localization pathway. IMAGES:

本研究通过分析携带缺失突变的α2-β-半乳糖苷酶融合蛋白（alpha 2-beta-galactosidase hybrid proteins），证实酵母阻遏蛋白α2（yeast repressor alpha 2）拥有两个结构迥异的核定位信号（nuclear localization signals）。通过对固定完整酵母细胞进行间接免疫荧光染色与光学切片成像，明确了各融合蛋白的细胞定位情况。

这两个核定位信号在α2的一级结构中相距甚远，且无任何序列同源性。其中一个信号如既往研究报道，位于α2的氨基末端13个氨基酸残基区段内；仅缺失该氨基末端信号，对核定位无显著影响。

第二个信号位于α2的中央区域，即α2同源结构域（homeodomain）内部。由于该信号处于α2同源结构域的氨基末端区段，因此该同源结构域除介导DNA结合功能外，还可独立完成核定位过程。

仅缺失第二个信号会导致突变蛋白定位效率低下，并使其在核膜的离散位点（推测为核孔（nuclear pore））处异常聚集。

我们提出，α2所含的两个核定位信号功能各异，在核定位通路的不同步骤中发挥作用。IMAGES: