Genetic Studies of ASK Gene Functions

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Plant development requires the coordinated activity of many gene products, which can be regulated at both the levels of protein synthesis and degradation. Many regulatory genes controlling flower development encode putative transcription factors, but few are known to regulate proteolysis. Work supported by an NSF grant indicates that the Arabidopsis ASK1 gene regulates vegetative and flower development and encodes a homolog of the yeast and human SKP1 protein. SKP1 is a subunit of SCF ubiquitin ligase complexes that catalyze the third step of a pathway that covalently links ubiquitins to target proteins, mediating the degradation of proteins. ASK1 interacts with UFO genetically to regulate flower development and the expression of the floral genes APETALA3 (AP3) and PISTILLATA (PI). The working model is that UFO and ASK1 are components of a SCF ubiquitin ligase that facilitates the degradation of a negative regulator of AP3 and PI expression. Furthermore, ASK1 is essential for homolog separation and segregation during male meiosis I. In meiosis I, specific proteins must be degraded at the metaphase/anaphase transition to allow the separation of homologs. Therefore, the simplest hypothesis is that ASK1 regulates such a proteolytic event. Arabidopsis possesses at least 20 SKP1 homologs, named ASKs. The functions of the other ASK genes are not known. The fact that the yeast SKP1 gene is essential for mitotic growth suggests that other ASK gene(s) probably have redundant functions that together control essential cellular processes. Therefore, understanding other ASK gene functions should provide new insights into the role of regulated proteolysis during development. In this project, the ASK1 function in regulating flower development will be further analyzed using genetic and molecular approaches, including the interaction between UFO and ASK1 in the context of regulation of AP3 and PI expression. In addition, the role of ASK1 in controlling homolog separation in meiosis I will be investigated using microscopic procedures. Moreover, additional ASK genes will be characterized by expression studies, by functional comparison with ASK1, and some of those ASK genes expressed in the flowers will be tested for function using reverse genetic approaches. These experiments will likely provide new insights about how plant reproductive development is regulated by genes, potentially benefiting agriculture and the environment. The study of gene functions when the Arabidopsis sequencing has is just been completed is both timely and beneficial. The results from these experiments will also likely provide the starting points for future research by many in the community of plant scientists as well as those studying non-plant organisms.