Tirtiaux, C. | Search bioRxiv

bioRxiv2026

Characterization of starch metabolism pathways in dinoflagellates, chromerids and apicomplexans highlights the presence of a starch accumulating symbiont in their common ancestor

Unlike most other heterotrophic protist lineages which synthesize glycogen, alveolates accumulate -glucan storage polysaccharides in the form of starch. Within Alveolata, ciliates do so by using a simple pathway analogous to that used for glycogen metabolism in eukaryotes. However, Myzozoa have evolved five additional enzymatic steps which are common to Dinoflagellata, Chromerids and Apicomplexa. Moreover, dinoflagellates have acquired seven additional enzymes that distinguish them from the Chromerids-Apicompexa lineages, while chromerids possess only a single enzyme not found in dinoflagellates. Such modifications are not found in other eukaryotic lineages save for those resulting directly from primary or secondary plastid endosymbiosis. The many differences affecting the starch pathway of Myzozoa can be explained by a single secondary or tertiary plastid endosymbiosis of a starch-containing alga followed by distinct compartmentalisation outcomes of starch metabolism in Dinoflagellata relative to chromerids and Apicomplexa.

evolutionary biology↗

bioRxiv2020

Substitution of nucleotide-sugar by trehalose-dependent glycogen synthesis pathways in Chlamydiales underlines an unusual requirement for storage polysaccharides within obligate intracellular bacteria

All obligate intracellular pathogens or symbionts of eukaryotes lack glycogen metabolism. Most members of the Chlamydiales order are exceptions to this rule as they contain the classical GlgA-GlgC-dependent pathway of glycogen metabolism that relies on the ADP-Glucose substrate. We surveyed the diversity of Chlamydiales and found glycogen metabolism to be universally present with the important exception of Criblamydiaceae and Waddliaceae families that had been previously reported to lack an active pathway. However, we now find elements of the more recently described GlgE maltose-1-P-dependent pathway in several protist-infecting Chlamydiales. In the case of Waddliaceae and Criblamydiaceae, the substitution of the classical pathway by this recently proposed GlgE pathway was essentially complete as evidenced by the loss of both GlgA and GlgC. Biochemical analysis of recombinant proteins expressed from Waddlia chondrophila and Estrella lausannensis established that both enzymes do polymerize glycogen from trehalose through the production of maltose-1-P by TreS-Mak and its incorporation into glycogens outer chains by GlgE. Unlike Mycobacteriaceae where GlgE-dependent polymerization is produced from both bacterial ADP-Glc and trehalose, glycogen synthesis seems to be entirely dependent on host supplied UDP-Glc and Glucose-6-P or on host supplied trehalose and maltooligosaccharides. These results are discussed in the light of a possible effector nature of these enzymes, of the chlamydial host specificity and of a possible function of glycogen in extracellular survival and infectivity of the chlamydial elementary bodies. They underline that contrarily to all other obligate intracellular bacteria, glycogen metabolism is indeed central to chlamydial replication and maintenance.

microbiology↗