Macromolecular interactions in the nucleoporin p62 complex of rat nuclear pores: binding of nucleoporin p54 to the rod domain of p62

Nuclear pore complexes are constructed from a large number of different proteins, called collectively nucleoporins. One of these nucleoporins, p62, has an alpha-helical coiled-coil COOH-terminal rod domain linked to an NH2-terminal domain that contains a series of degenerate pentapeptide repeats. In nuclear pores p62 forms a tight complex with at least two other proteins, p58 and p54, which can be extracted from isolated rat liver nuclei (Finlay, D. R., E. Meier, P. Bradley, J. Horecka, and D. J. Forbes. 1991. J. Cell Biol. 114:169-183). We have used a range of methods to demonstrate a strong binding between p62 and p54 in this complex and show that the rod domain of p62 appears to constitute the principal binding site for p54. Whole p62 and its rod domain expressed in Escherichia coli both bind strongly to p54 in blot-overlay assays. Most of the epitopes on the p62 rod recognized by polyclonal antisera are masked in the complex, whereas epitopes on the NH2-terminal domain of p62 are still exposed, both in the isolated complex and also in nuclear pores stained in situ by immunofluorescence in isolated rat nuclei. Moreover, it has been possible to exchange recombinant p62 rod for some of the native p62 in complexes partially dissociated by 4 M urea. Overall these results suggest a key role for the p62 rod domain in maintaining the structural integrity of the complex and also suggest a molecular model for the complex. This model is consistent with data that indicate that the analogous coiled-coil region of yeast nucleoporin NSP1 may function in a similar way.