Evolution has enlisted a large variety of posttranslational modifications to provide temporal, spatial and functional regulation of the protein machinery of the cell. This project focuses on a specific example of a type that has seemingly been borrowed from the secretory pathway of eukaryotic cells, glycosylation, but might actually have first evolved in the cytoplasm of bacterial cells. We propose that complex cytoplasmic glycosylation exerts unique glycoregulatory functions in eukaryotes, and is subject to distinct controls relative to `conventional'protein glycosylation in the secretory pathway. The initial organism of analysis is the social amoeba Dictyostelium, and the target of the pathway studied here is Skp1, an adaptor of the SCF class of E3 ubiquitin ligases whose targets are frequently activated by phosphorylation and for which there may be a need for an independent mode of covalent regulation. Most remarkable is that this modification involves six enzymatic steps resulting in the assembly of a pentasaccharide attached to a highly conserved residue of proline. This modification, with a structural richness rivaling that of a peptide, is hypothesized to target only Skp1 and modulate its regulation of a critical developmental transition (culmination). Genetic manipulation of prolyl hydroxylase expression controls the O2-requirement for development suggesting a normal role for this enzyme in O2-regulation. Recent analysis of the effects of disrupting enzyme genes required for the sequential hydroxyproline-dependent glycosylation of Skp1 gives evidence for additional levels of hierarchical regulation of O2-dependent development, which is to be characterized in this project. Our recent discovery of the last enzyme (AgtA) needed to construct the pentasaccharide has positioned us finally to address these ideas genetically and biochemically. At the outset, we will in aim 1 define the linkages of the two ?-linked galactose sugars whose additions appear to be catalyzed by AgtA, which will enable chemical synthesis of the glycan for the later aims.
Aim 2 will examine the basis for apparent AgtA processivity in adding the two sugars, and how Skp1 and the catalytic and ?-propeller-like domains of AgtA mutually regulate each other's activity, hypothesized to be associated with quality control.
Aim 3 will employ reverse genetic and epistatic analysis of the glycosylation genes to test whether hierarchical regulation is linear or involves parallel signaling pathways. In addition, new antibodies will be developed to monitor progressive variations in Skp1 glycosylation in the cells which signal development. Finally, to identify the functionally most important features of the modification pathway, aim 4 will carry out tests for the evolutionary conservation of Skp1 glycoregulation in the apicomplexan Toxoplasma gondii, the agent for human toxoplasmosis. The knowledge gained is expected to generate new ideas of how the proteome is regulated in select protists in response to external signals such as O2 and internal signals such as sugar metabolites.
MEDICAL RELEVANCE The study utilizes the NIH model organism Dictyostelium discoideum, a social amoeba allied genomically with the human parasite Entamoeba. Dictyostelium, which offers special advantages for molecular, biochemical and cell biological studies owing to its free-living, wall-less lifestyle and haploid genome, has proven to be a useful model for select pathways of protein glycosylation of various other types of protists. One example is the cytoplasmic glycosylation pathway examined in this investigation. Bioinformatics and early biochemical studies indicate that the main part of the pathway is present in the large Phytophthora group of plant pathogens, the agent for human toxoplasmosis Toxoplasma gondii, and other protists. Since T. gondii is an intracellular pathogen, Dictyostelium offers an attractive surrogate host for the biochemical and reverse genetic analysis of the Toxoplasma genes that are candidates for the Skp1 modification pathway in this organism. This will serve as a prelude for future direct studies on the function of the pathway for O2-regulation of T. gondii, which is widely disseminated latently in the human population and for which, if it is re-activated, pharmacological therapies are extremely limited.
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