Convergent evolution of polysaccharide debranching defines a common mechanism for starch accumulation in cyanobacteria and plants.

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.

Preparation and enzymatic hydrolysis of nanoparticles made from single xyloglucan polysaccharide chain.

In this work, polysaccharide nanoparticles based on tamarind seeds xyloglucan are prepared, analyzed in term of characteristic sizes and morphology, and degraded by the action of a glycoside-hydrolase. Obtained in an aqueous NaNO2 solution (0.1M), these unaggregated nanoparticles have a characteristic diameter of ca. 60 nm (DLS, AFM and TEM measures). They are not compact, but highly swollen and look like hyperbranched and dendrimer-like (soft sphere model) structures. This observation is coherent with the native structure of the xyloglucan macromolecules which are themselves branched. The enzymatic hydrolysis by cellulase of Trichoderma reesei of the xyloglucan nanoparticles is investigated. In particular, the apparent mass molecular weight drastically decreases meaning that the xyloglucan nanoparticles are effectively fully hydrolyzed by the endo-ß-(1,4)-glucanase. Furthermore, we observe that the enzyme has to uncoil the nanoparticles before cutting the ß-(1→4) bonds and digesting the xyloglucan.

Genetic dissection of floridean starch synthesis in the cytosol of the model dinoflagellate Crypthecodinium cohnii.

Starch defines an insoluble semicrystalline form of storage polysaccharides restricted to Archaeplastida (red and green algae, land plants, and glaucophytes) and some secondary endosymbiosis derivatives of the latter. While green algae and land-plants store starch in plastids by using an ADP-glucose-based pathway related to that of cyanobacteria, red algae, glaucophytes, cryptophytes, dinoflagellates, and apicomplexa parasites store a similar type of polysaccharide named floridean starch in their cytosol or periplast. These organisms are suspected to store their floridean starch from UDP-glucose in a fashion similar to heterotrophic eukaryotes. However, experimental proof of this suspicion has never been produced. Dinoflagellates define an important group of both photoautotrophic and heterotrophic protists. We now report the selection and characterization of a low starch mutant of the heterotrophic dinoflagellate Crypthecodinium cohnii. We show that the sta1-1 mutation of C. cohnii leads to a modification of the UDP-glucose-specific soluble starch synthase activity that correlates with a decrease in starch content and an alteration of amylopectin structure. These experimental results validate the UDP-glucose-based pathway proposed for floridean starch synthesis.