Connections in yeast cell polarity.

Genetics and biochemistry have been used to map many of the individual pathways that establish and maintain cell polarity in yeast, but Drees et al. (page 549) have now produced the equivalent of an aerial photograph of these processes. Using a high-throughput yeast two-hybrid screen, the authors assayed the universe of likely protein–protein interactions involved in cell polarity development. The resulting protein interaction map provides tantalizing insights and identifies dozens of potential mechanistic connections worth closer examination.

The authors used 68 yeast proteins associated with the actin cytoskeleton, septins, the secretory apparatus, and Rho-type GTPases as baits in parallel two-hybrid screens covering ∼90% of the predicted Saccharomyces cerevisiae ORFs. The screen uncovered 128 novel protein–protein interactions, including 44 involving previously uncharacterized proteins. The appearance of known interactions in the screen, along with subcellular localization studies, suggests that many of the newly identified interactions are relevant in vivo.

Many of the interactions help to explain the phenotypes of previously described yeast mutants. For example, multicopy expression of Msb2 suppresses the phenotype of a Cdc24 mutant, but the function of Msb2 remains unknown. The new analysis identifies a two-hybrid interaction between Cla4 and Msb2, suggesting that Msb2, like Cdc24, is directly involved in the Cdc42 pathway. Other interactions imply new links between known pathways, such as between the secretory pathway regulated by Rho1 and the Cdc42 pathway, which is essential for establishing and maintaining cell polarity. Still other interactions suggest direct connections between actin assembly and the morphogenesis checkpoint.

Because the machinery of cell polarity development is highly conserved from yeast to humans, the newly described interactions merit further study in a variety of cell types. The authors stress that the screen was not exhaustive, so additional interactions certainly await discovery. ▪