Characterization of function of the GlgA2 glycogen/starch synthase in cyanobacterium sp. Clg1 highlights convergent evolution of glycogen metabolism into starch granule aggregation

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

International audienceAll 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 glycogen’s 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

Characterization of Function of the GlgA2 Glycogen/Starch Synthase in Cyanobacterium

At variance with the starch-accumulating plants and most of the glycogen-accumulating cyanobacteria, Cyanobacterium sp. CLg1 synthesizes both glycogen and starch. We now report the selection of a starchless mutant of this cyanobacterium that retains wild-type amounts of glycogen. Unlike other mutants of this type found in plants and cyanobacteria, this mutant proved to be selectively defective for one of the two types of glycogen/starch synthase: GlgA2. This enzyme is phylogenetically related to the previously reported SSIII/SSIV starch synthase that is thought to be involved in starch granule seeding in plants. This suggests that, in addition to the selective polysaccharide debranching demonstrated to be responsible for starch rather than glycogen synthesis, the nature and properties of the elongation enzyme define a novel determinant of starch versus glycogen accumulation. We show that the phylogenies of GlgA2 and of 16S ribosomal RNA display significant congruence. This suggests that this enzyme evolved together with cyanobacteria when they diversified over 2 billion years ago. However, cyanobacteria can be ruled out as direct progenitors of the SSIII/SSIV ancestral gene found in Archaeplastida. Hence, both cyanobacteria and plants recruited similar enzymes independently to perform analogous tasks, further emphasizing the importance of convergent evolution in the appearance of starch from a preexisting glycogen metabolism network