Abstract. Nuplp is a yeast nuclear pore complex protein (nucleoporin) required for nuclear protein import, mRNA export and maintenance of normal nuclear architecture. We have used a genetic approach to identify other proteins that interact functionally with Nuplp. Here we describe the isolation of seventeen mutants that confer a requirement for Nuplp in a background in which this protein is normally not essential. Some of the mutants require wild-type Nuplp, while others are viable in combination with specific nupl alleles. Several of the mutants show nonallelic noncomplementation, suggesting that the products may be part of a hetero-oligomeric complex. One is allelic to srpl which, although it was identified in an unrelated screen, was shown to encode a protein that is localized to the nuclear envelope (Yano, R., M. Oakes, M. Yamaghishi, J. A. Dodd, and M. Nomura. 1992. Mol. Cell. Biol. 12:5640-5651). We have used immunoprecipitation and fusion protein precipitation to show that Srplp forms distinct complexes with both Nuplp and the related nucleoporin Nup2p, indicating that Srplp is a component of the nuclear pore complex. The distant sequence similarity between Srplp and the 13-catenin/desmoplakin family, coupled with the altered structure of the nuclear envelope in nupl mutants, suggests that Srplp may function in attachment of the nuclear pore complex to an underlying nuclear skeleton.

The importin ␣⅐␤ heterodimer mediates nuclear import of proteins containing classical nuclear localization signals. After carrying its cargo into the nucleus, the importin dimer dissociates, and Srp1p (the yeast importin ␣ subunit) is recycled to the cytoplasm in a complex with Cse1p and RanGTP. Nup2p is a yeast FXFG nucleoporin that contains a Ran-binding domain. We find that export of Srp1p from the nucleus is impaired in ⌬nup2 mutants. Also, Srp1p fusion proteins accumulate at the nuclear rim in wild-type cells but accumulate in the nuclear interior in ⌬nup2 cells. A deletion of NUP2 shows genetic interactions with mutants in SRP1 and PRP20, which encodes the Ran nucleotide exchange factor. Srp1p binds directly to an Nterminal domain of Nup2p. This region of Nup2p is sufficient to allow accumulation of an Srp1p fusion protein at the nuclear rim, but the C-terminal Ran-binding domain of Nup2p is required for efficient Srp1p export.