Formation of nuclear matrix filaments by p27^BBP/eIF6

p27^BBP/eIF6 is an evolutionarily conserved protein necessary for ribosome biogenesis which was cloned in mammals for its ability to bind the cytodomain of β4 integrin. In cultured cells, a conspicuous fraction of p27^BBP/eIF6 is associated with the intermediate filaments/nuclear matrix (IF/NM) cytoskeleton. The mechanism of this association is not known. Here we show that in epidermis p27^BBP/eIF6 is naturally associated with IF/NM. To analyze the intrinsic capability of p27^BBP/eIF6 to generate cytoskeletal networks, the properties of the pure, recombinant, untagged protein were studied. Recombinant p27^BBP/eIF6 binds β4 integrin. Upon dialysis against IF buffer, p27^BBP/eIF6 forms polymers which, strikingly, have a morphology identical to NM filaments. Cross-linking experiments suggested that polymerization is favored by the formation of disulphide bridges. These data suggest that p27^BBP/eIF6 is associated with the cytoskeleton, and contributes to formation of NM filaments. These findings help to settle the controversy on nuclear matrix.