Synergy of importin α recognition and DNA binding by the yeast transcriptional activator GAL4

The N-terminus of the yeast transcriptional activator GAL4 contains partially overlapping nuclear targeting and DNA binding functions. We have previously shown that GAL4 is recognised with high affinity by importin β and not by the conventional nuclear localisation sequence binding importin α subunit of the importin α/β heterodimer. The present study uses ELISA-based binding and electrophoretic mobility shift assays to show that recognition of GAL4 by importin α can occur, but only when GAL4 is bound to its specific DNA recognition sequence. Intriguingly, binding by importin α enhances DNA binding on the part of GAL4, implying a synergistic co-operation between these two functions. The results implicate a possible role for importin α in the nucleus additional to its established role in nuclear transport, as well as having implications for the use of GAL4 as a DNA carrier in gene therapy applications.