Cross-species comparisons in a unified medium suggest broadly stable glycosome-linked enzyme levels under nutrient and oxygen variation.

Abstract

Introduction: Kinetoplastids are one of two groups of euglenozoans that have compartmentalized the enzymes of metabolic pathways, including glycolysis, into a peroxisome-derived organelle called a glycosome. Several ideas have emerged regarding the evolutionary pressures that drove and are maintaining compartmentalization of these typically cytosolic enzymes. For example, their compartmentalization might result in superior efficiency in remodeling enzyme relative abundances in response to changing environmental conditions. We wished to empirically test the merits of this explanation. We began with the presumption that the abundances of glycosome-compartmentalized enzymes are broadly affected by changes in extracellular environment, similar to how they vary between life stages of the model kinetoplastid Trypanosoma brucei. Our hypothesis was that abundances of glycosome-localized enzymes in multiple kinetoplastids would alter as a result of their extracellular environment.

Methods: Six different kinetoplastid species, including both dixenous and monoxenous parasites, were evaluated in culture for their response to extracellular nutrient and oxygen availability in terms of their expression of glycosome-localized enzymes. We analyzed their growth rates and utilized immunoblotting to measure abundances of glycosome-localized or partially-localized enzymes and cytosolic enolase in the different culture conditions.

Results: We found the tested enzyme abundances to be largely consistent. Oxygen availability and slow removal of nutrients from the extracellular environment resulted in only minor changes in enzyme abundance. Only abrupt replenishment of culture nutrients resulted in enzyme abundance responses - typically a modest decrease in both cytosolic enolase and the glycosome-localized enzymes.

Discussion: Results suggest that there is little evidence that parasites respond to external nutrient or oxygen availability through alterations in relevant enzyme abundances, at least absent life cycle transitions. Thus, our study does not provide support for the idea that efficient and coordinated remodeling of ratios of metabolic enzymes was an evolutionary driver of metabolic enzymes' compartmentalization in the glycosome. Further, it suggests that it may be relevant to test whether posttranslational modifications and/or changes to enzyme localization are mechanisms trypanosome utilize to adjust specific metabolic activity when needed.