A novel complex of membrane proteins required for formation of a sphericalnucleus

Two membrane proteins were identified through their genetic interaction wit h the nucleoporin Nup84p and shown to participate in nuclear envelope morph ogenesis in yeast. One component is a known sporulation factor Spo7p, and t he other, Nem1p, a novel protein whose C-terminal domain is conserved durin g eukaryotic evolution. Spo7p and Nem1p localize to the nuclear/ER membrane and behave biochemically as integral membrane proteins, Nem1p binds to Spo 7p via its conserved C-terminal domain. Although cells without Spo7p or Nem 1p are viable, they exhibit a drastically altered nuclear morphology with l ong, pore-containing double nuclear membrane extensions. These protrusions emanate from a core nucleus which contains the DNA, and penetrate deeply in to the cytoplasm, Interestingly, not only Spo7(-) and Nem1(-), but also sev eral nucleoporin mutants are defective in sporulation, Thus, Spo7p and Nem1 p, which exhibit a strong genetic link to nucleoporins of the Nup84p comple x, fulfil an essential role in formation of a spherical nucleus and meiotic division.