A method for the large-scale cloning of nuclear proteins and nuclear targeting sequences on a functional basis

We describe here a selection strategy allowing the cloning of sequences that contain a functional nuclear targeting signal. Our method relies on the use of green fluorescent protein fusion proteins to identify nuclear targeting sequences. Transfected cells expressing nuclear protein fusions were isolated on the basis of their nuclear fluorescence using flow cytometry and the transfected DNAs were recovered after bacterial transformation with total DNA from pools of sorted cells. Starting from a cDNA expression library, in which only 1% of the expressed proteins were nuclear, we obtained a 70-fold enrichment in nuclear protein-encoding clones after a single round of selection. Among the 63 clones that have been partially sequenced to date, 25 (40%) corresponded to known nuclear proteins and 13 (20%) to previously uncharacterized sequences. Despite their ability to target the green fluorescent protein marker to the cell nucleus, about half of the cloned sequences did not encode canonical basic or bipartite nuclear localization signals. The method can thus be applied to the large-scale cloning of functional nuclear targeting sequences, which opens the way to a wide investigation of nuclear import mechanisms and to the identification of previously unknown nuclear proteins.