The Dictyostelium prestalk cell inducer DIF regulates nuclear accumulation of a STAT protein by controlling its rate of export from the nucleus.

becomes tyrosine phosphorylated, dimerises and accumulates in the nuclei of Dictyostelium cells exposed to DIF, the chlorinated hexaphenone that directs prestalk cell differentiation. By performing cytoplasmic photobleaching of living cells, we show that DIF inhibits the nuclear export of Within Dd-duanyu1813c there is a 50 amino acid region containing several consensus CRM1 (exportin 1)-dependent nuclear export signals (NESs). Deletion of this region causes Dd-duanyu1813c to accumulate in the nucleus constitutively and, when coupled to GFP, the same region directs nuclear export. We show that the N-terminal-proximal 46 amino acids are necessary for nuclear accumulation of Dd-duanyu1813c and sufficient to direct constitutive nuclear accumulation when fused to GFP. Combining the photobleaching and molecular analyses, we suggest that DIF-induced dimerisation of Dd-duanyu1813c functionally masks the NES-containing region and that this leads to nett nuclear accumulation, directed by the N-terminal-proximal import signals. These results show that the regulated nuclear accumulation of a protein can be controlled at the level of nuclear export and they also provide a better understanding of the mechanism whereby DIF directs cell type divergence.

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