Many neuro-degenerative and metabolic diseases like Parkinson’s and Alzheimer’s are attributed to the effect of mis-folded and aggregated state of proteins in cells. This suggests that the phenomenon of in vivo protein aggregation may be relatively common, perhaps more than currently appreciated. In this study, we aimed to decipher the cause behind an intriguing and potentially related phenomenon observed in yeast cells - a widespread reorganization of hundreds of cytosolic proteins into punctate foci under starvation conditions. The key question that emerges is whether this phenomenon represents organization of proteins into functional assemblies or catastrophic aggregation. This thesis supports the aggregation hypothesis and provides evidence of its role in shaping the dynamics of cellular proteomes. We have been able to demonstrate that the proteins forming foci share a high propensity to aggregate and that these foci may represent sites of homogenous protein aggregation, structures which are typically associated with chaperones. A link between the formation of foci to the yeast aging process has also been established. With evidence correlating protein aggregation propensities to the cellular energy state, we have extended the current "living on the edge" hypothesis (which demonstrates an inverse correlation between protein expression levels and their aggregation propensities). For a specific case of the "purinosome", which is inferred to be a functional enzyme complex responsible for purine biosynthesis, we have shown that the observations may be explained alternatively as a generic protein aggregation phenomenon. This study highlights a systems approach to studying cellular proteins, which can corroborate or provide an alternative explanation to inferences drawn from traditional reductionistic analysis. / text

Purines are organic compounds with miscellaneous functions that are found in all living organisms in complex molecules such as nucleotides, nucleosides or as purine bases. The natural balance of purine levels is maintained by their synthesis, recycling and degradation. Excess purines are excreted in the urine as uric acid. Purine nucleotides may be recycled by salvage pathways catalysing the reaction of purine base with phosphoribosyl pyrophosphate. A completely new central molecule of purine metabolism, inosine monophosphate, can be synthesized from precursors during the de novo purine synthesis (DNPS). DNPS involves ten steps catalysed by six enzymes that form a multienzymatic complex, the purinosome, enabling substrate channelling through the pathway. DNPS is activated under conditions involving a high purine demand such as organism development. Currently, three DNPS-disrupting disorders have been described: ADSL deficiency, AICA-ribosiduria and PAICS deficiency. All three disorders are caused by genetic mutations leading to the impaired function of particular enzyme causing insufficient activity of respective DNPS step, manifested biochemically by accumulation of substrate of deficient enzyme, biologically by disruption of purinosome formation and clinically by unspecific neurological features,...