Genetic and biochemical studies in lower eukaryotes have identified several proteins that ensure accurate segregation of chromosomes. These include the Drosophila aurora and yeast Ipl1 kinases that are required for centrosome maturation and chromosome segregation. We have identified two human homologues of these genes, termed aurora1 and aurora2, that encode cell-cycle-regulated serine/threonine kinases. Here we demonstrate that the aurora2 gene maps to chromosome 20q13, a region amplified in a variety of human cancers, including a significant number of colorectal malignancies. We propose that aurora2 may be a target of this amplicon since its DNA is amplified and its RNA overexpressed, in more than 50% of primary colorectal cancers. Furthermore, overexpression of aurora2 transforms rodent fibroblasts. These observations implicate aurora2 as a potential oncogene in many colon, breast and other solid tumors, and identify centrosome-associated proteins as novel targets for cancer therapy.

The Aurora kinase Ipl1p plays a crucial role in regulating kinetochore-microtubule attachments in budding yeast, but the underlying basis for this regulation is not known. To identify Ipl1p targets, we first purified 28 kinetochore proteins from yeast protein extracts. These studies identified five previously uncharacterized kinetochore proteins and defined two additional kinetochore subcomplexes. We then used mass spectrometry to identify 18 phosphorylation sites in 7 of these 28 proteins. Ten of these phosphorylation sites are targeted directly by Ipl1p, allowing us to identify a consensus phosphorylation site for an Aurora kinase. Our systematic mutational analysis of the Ipl1p phosphorylation sites demonstrated that the essential microtubule binding protein Dam1p is a key Ipl1p target for regulating kinetochore-microtubule attachments in vivo.

Phosphorylation is a universal mechanism for regulating cell behavior in eukaryotes. Although protein kinases are known to target short linear sequence motifs on their substrates, the rules for kinase substrate recognition are not completely understood. We used a rapid peptide screening approach to determine consensus phosphorylation site motifs targeted by 61 of the 122 kinases in Saccharomyces cerevisae. Correlation of these motifs with kinase primary sequence has uncovered previously unappreciated rules for determining specificity within the kinase family, including a residue determining P−3 Arg specificity among members of the CMGC group of kinases. Furthermore, computational scanning of the yeast proteome enabled the prediction of thousands of new kinase-substrate relationships. We experimentally verified several candidate substrates of the Prk1 family of kinases in vitro and in vivo, and we identified a protein substrate of the kinase Vhs1. Together, these results elucidate how kinase catalytic domains recognize their phosphorylation targets and suggest general avenues for the identification of new kinase substrates across eukaryotes.

The enzyme that catalyzes the synthesis of the major structural component of the yeast cell wall, beta(1-->3)-D-glucan synthase (also known as 1,3-beta-glucan synthase), requires a guanosine triphosphate (GTP) binding protein for activity. The GTP binding protein was identified as Rho1p. The rho1 mutants were defective in GTP stimulation of glucan synthase, and the defect was corrected by addition of purified or recombinant Rho1p. A protein missing in purified preparations from a rho1 strain was identified as Rho1p. Rho1p also regulates protein kinase C, which controls a mitogen-activated protein kinase cascade. Experiments with a dominant positive PKC1 gene showed that the two effects of Rho1p are independent of each other. The colocalization of Rho1p with actin patches at the site of bud emergence and the role of Rho1p in cell wall synthesis emphasize the importance of Rho1p in polarized growth and morphogenesis.

Many genes required for cell polarity development in budding yeast have been identified and arranged into a functional hierarchy. Core elements of the hierarchy are widely conserved, underlying cell polarity development in diverse eukaryotes. To enumerate more fully the protein–protein interactions that mediate cell polarity development, and to uncover novel mechanisms that coordinate the numerous events involved, we carried out a large-scale two-hybrid experiment. 68 Gal4 DNA binding domain fusions of yeast proteins associated with the actin cytoskeleton, septins, the secretory apparatus, and Rho-type GTPases were used to screen an array of yeast transformants that express ∼90% of the predicted Saccharomyces cerevisiae open reading frames as Gal4 activation domain fusions. 191 protein–protein interactions were detected, of which 128 had not been described previously. 44 interactions implicated 20 previously uncharacterized proteins in cell polarity development. Further insights into possible roles of 13 of these proteins were revealed by their multiple two-hybrid interactions and by subcellular localization. Included in the interaction network were associations of Cdc42 and Rho1 pathways with proteins involved in exocytosis, septin organization, actin assembly, microtubule organization, autophagy, cytokinesis, and cell wall synthesis. Other interactions suggested direct connections between Rho1- and Cdc42-regulated pathways; the secretory apparatus and regulators of polarity establishment; actin assembly and the morphogenesis checkpoint; and the exocytic and endocytic machinery. In total, a network of interactions that provide an integrated response of signaling proteins, the cytoskeleton, and organelles to the spatial cues that direct polarity development was revealed.