A 55-kDa protein (p55) of the goat uterus mediates nuclear transport of the estrogen receptor. I. Purification and characterization.

A 55-kDa protein (p55), purified from the goat uterine cytosol, transports estrogen receptor (ER) into the nucleus. Selective elution of this protein from a column of estrogen receptor-Sepharose using buffers containing high concentrations of lysine and the high affinity with which it binds to poly-L-lysine-Sepharose indicate that it recognizes a lysine-rich region in the ER. Its strong binding to tubulin-Sepharose and actin-Sepharose is indicative of a role that the cytoskeletal elements play in the nuclear transport of the ER, mediated by p55. This protein can be purified in a single step following chromatography of the uterine cytosol on a column of actin-Sepharose. Antibodies raised against poly-L-aspartic acid cross-reacted with p55 and inhibited the nuclear transport of the ER. The binding of p55 to a heterologous nuclear localization sequence of the SV40 large T suggests that it may also be involved in the transport of proteins other than the ER.