Convergent Evolution of Polysaccharide Debranching Defines a Common Mechanism for Starch Accumulation in Cyanobacteria and Plants

Abstract

Starch, unlike hydrosoluble glycogen particles, aggregates into insoluble, semicrystalline granules. In photosynthetic eukaryotes, the transition to starch accumulation occurred after plastid endosymbiosis from a preexisting cytosolic host glycogen metabolism network. This involved the recruitment of a debranching enzyme of chlamydial pathogen origin. The latter is thought to be responsible for removing misplaced branches that would otherwise yield a water-soluble polysaccharide. We now report the implication of starch debranching enzyme in the aggregation of semicrystalline granules of single-cell cyanobacteria that accumulate both glycogen and starch-like polymers. We show that an enzyme of analogous nature to the plant debranching enzyme but of a different bacterial origin was recruited for the same purpose in these organisms. Remarkably, both the plant and cyanobacterial enzymes have evolved through convergent evolution, showing novel yet identical substrate specificities from a preexisting enzyme that originally displayed the much narrower substrate preferences required for glycogen catabolism.

Characterization of Function of the GlgA2 Glycogen/Starch Synthase in Cyanobacterium sp. Clg1 Highlights Convergent Evolution of Glycogen Metabolism into Starch Granule Aggregation

“…CLg1. Interestingly, similar increases have been noted in other glycogen metabolism cyanobacterial mutants by others (Fu and Xu, 2006;Cenci et al, 2013).…”

“…After 3 years of segregation and phenotype screening, we selected seven strains defining the class C mutants, which contained water-soluble polysaccharides (WSPs) in amounts close to those of the wild-type reference but with significantly lower amounts of starch. Six of these seven mutants were reported previously by Cenci et al (2013) but failed to reveal the biochemical explanation for the mutant phenotype. The seventh strain (187G11) displayed a very severe phenotype defined by the absence of iodine stain after spraying cell patches with iodine vapors.…”

“…Cyanobacterium sp. CLg1 has been reported to accumulate both a major starch fraction and a minor yet significant glycogen pool (Falcon et al, 2004;Cenci et al, 2013). We now report the selection of a starchless mutant of Cyanobacterium sp.…”

“…We previously published that mutants defective for a debranching enzyme (GlgX2) overaccumulated glycogen and witnessed a dramatic decrease in starch amounts. To make sure that the phenotype displayed in 187G11 could not result from the combination of a direct effect on the glycogen/starch synthase and an indirect effect on the GlgX2 debranching enzyme, we semiquantified GlgX2 by zymogram analysis through the procedures detailed by Cenci et al (2013) and found the activity to be normal qualitatively and quantitatively (Supplemental Fig. S3).…”

“…A collection of 2 3 10 4 mutants was generated after UV light mutagenesis followed by a minimum of four rounds of subcloning, as detailed previously by Cenci et al (2013). Following the first round of screening, the selected mutants were further subcloned to check for complete segregation of the mutant phenotype.…”