Combining with a nuclear export signal (NES) on a cargo to be transported, to mediate transport of a the cargo through the nuclear pore, from the nuclear lumen to the cytoplasm. The cargo can be either a RNA or a protein. [ http://en.wikipedia.org/wiki/Nuclear_transport http://www.ncbi.nlm.nih.gov/pubmed/11743003 http://www.ncbi.nlm.nih.gov/pubmed/28713609 http://www.ncbi.nlm.nih.gov/pubmed/25802992 ]