Alterations in nuclear structure are associated with aging and cancer, and yet the functional link between nuclear architecture and nuclear functions is not well understood. We have been using budding yeast as a model system to study nuclear architecture. In an earlier study from our lab (Campbell et al, 2006), we showed that the shape of the yeast nucleus is affected by processes that regulate lipid biosynthesis. In particular, inactivating the yeast homolog of lipin leads to expansion of the nuclear membrane in a particular nuclear compartment. To further understand the link between lipid homeostasis and nuclear function we embarked on a genetic screen for mutations that render the lipin pathway essential for viability. To this end, we combined mutations in candidate genes with the spo7 mutation, looking for reduced growth in the double mutant. The candidate genes were selected based on their function and localization; most were involved in processes that take place near or at the nuclear membrane and most mutations were in genes coding for proteins that exhibit a peripheral localization. Through this screen we found a mutant that was known to affect spindle pole body function. The yeast spindle pole body is equivalent to the metazoan centrosome and it acts in nucleating spindle microtubules during chromosome segregation and in nucleating cytoplasmic microtubules that are needed for nuclear movement. The mechanism by which the spindle pole body is inserted and maintained in the nuclear membrane is not known, but this is a question of immense importance because the mechanism of nuclear membrane insertion could apply to other nuclear membrane structures, such as nuclear pore complexes. We found that additional mutations known to abrogate spindle pole body assembly also relay on the lipin pathway for viability. These findings suggest that the composition of the nuclear membrane affects the function of nuclear membrane associated structures. Our studies are now focused on understanding the nature of this defect, using cell biology and genetic methodologies. We are also collaborating with Dr. Sue Jaspersen in order to look at spindle pole bodies by electron microscopy and to determine the relationship between nuclear membrane composition and the assembly of integral membrane structures.
