Mutation of the yeast epsilon-COP gene ANU2 causes abnormal nuclear morphology and defects in intracellular vesicular transport

Previously we reported an original method of visualizing the shape of yeast nuclei by the expression of green fluorescent protein (GFP)-tagged Xenopus nucleoplasmin in Saccharomyces cerevisiae. To identify components that det ermine nuclear structure, we searched for mutants exhibiting abnormal nucle ar morphology from a collection of temperature-sensitive yeast strains expr essing GFP-tagged nucleoplasmin. Four anu mutant strains (anu1-1, 2-1, 3-1 and 4-1; ANU=abnormal nuclear morphology) that exhibited strikingly differe nt nuclear morphologies at the restrictive temperature as compared to the w ild-type were isolated. The nuclei of these mutants were irregularly shaped and often consisted of multiple lobes. ANU1, 3 and 4 were found to encode known factors Sec24p, Sec13p and Sec18p, respectively, all of which are inv olved in the formation or fusion of intracellular membrane vesicles of prot ein transport between the endoplasmic reticulum (ER) and the Golgi apparatu s. On the other hand, ANU2 was not well characterized. Disruption of ANU2 ( Delta anu2) was not lethal but conferred temperature-sensitivity for growth . Electron microscopic analysis of anu2-1 cells revealed not only the abnor mal nuclear morphology but also excessive accumulation of ER membranes. In addition, both anu2-1 and Delta anu2 cells were defective in protein transp ort between the ER and the Golgi, suggesting that Anu2p has an important ro le in vesicular transport in the early secretory pathway. Here we show that ANU2 encodes a 34 kDa polypeptide, which shares a 20% sequence identity wi th the mammalian epsilon-COP. Our results suggest that Anu2p is the yeast h omologue of mammalian epsilon-COP and the abrupt accumulation of the ER mem brane caused by a blockage of the early protein transport pathway leads to alteration of nuclear morphology of the budding yeast cells.