Nature of the Periplastidial Pathway of Starch Synthesis in the CryptophyteGuillardia theta

Abstract

The nature of the periplastidial pathway of starch biosynthesis was investigated with the model cryptophyte Guillardia theta. The storage polysaccharide granules were shown to be composed of both amylose and amylopectin fractions with a chain length distribution and crystalline organization very similar to those of starch from green algae and land plants. Most starch granules displayed a shape consistent with biosynthesis occurring around the pyrenoid through the rhodoplast membranes. A protein with significant similarity to the amylose-synthesizing granule-bound starch synthase 1 from green plants was found as the major polypeptide bound to the polysaccharide matrix. N-terminal sequencing of the mature protein proved that the precursor protein carries a nonfunctional transit peptide in its bipartite topogenic signal sequence which is cleaved without yielding transport of the enzyme across the two inner plastid membranes. The enzyme was shown to display similar affinities for ADP and UDP-glucose, while the V max measured with UDP-glucose was twofold higher. The granule-bound starch synthase from Guillardia theta was demonstrated to be responsible for the synthesis of long glucan chains and therefore to be the functional equivalent of the amylose-synthesizing enzyme of green plants. Preliminary characterization of the starch pathway suggests that Guillardia theta utilizes a UDP-glucose-based pathway to synthesize starch.

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

“…This may imply that starch granule aggregation could dispense with the postulated debranching mechanism, although the crystallinity of this residual material still needs to be ascertained. Second, some of the secondary endosymbiosis lineages derived from Archaeplastida, as well as some little studied semiamylopectin accumulating cyanobacteria, were reported to lack candidate-direct DBE genes (Deschamps et al, 2006;Ball et al, 2011;Colleoni and Suzuki, 2012;Curtis et al, 2012). This observation would imply a nonessential function of polysaccharide debranching in amylopectin synthesis.…”

“…Until recently, the distribution of starch seemed restricted to photosynthetic eukaryotes, including several secondary endosymbiosis lineages derived from Archaeplastida, such as the cryptophytes (Deschamps et al, 2006), the dinoflagellates (Dauvillée et al, 2009), and some apicomplexa parasites (Coppin et al, 2005). However, more recent studies revealed the existence of starch-like structures in unicellular diazotrophic cyanobacteria belonging to the order Chroococcales (Nakamura et al, 2005;Deschamps et al, 2008;Suzuki et al, 2013).…”

“…This observation would imply a nonessential function of polysaccharide debranching in amylopectin synthesis. Nevertheless, at least in one case, bona fide DBE activities were reported in biochemical assays, hinting that other glucan hydrolases may have evolved to ensure this function (Deschamps et al, 2006).…”

Algal genomes reveal evolutionary mosaicism and the fate of nucleomorphs

“…2.4.5) reveals many PPC-localized reactions catalysed by key enzymes such as glucan, water dikinase, alpha-amylase, hexokinase, 6-phosphofructokinase and phosphoglucomutase. These enzymes form a link to the synthesis and degradation of starch, which occurs in the PPC in G. theta 25 . Thirty-six candidate proteins for PPC, plastid or cytosol metabolite shuttling …”

Double Knockout Mutants of Arabidopsis Grown under Normal Conditions Reveal that the Plastidial Phosphorylase Isozyme Participates in Transitory Starch Metabolism

“…Initially, this enzyme was considered to be the main starch synthesizing activity (Hanes, 1940). Later, both starch and glycogen synthases have been discovered that utilize either ADPglucose or UDPglucose (or both; Deschamps et al, 2006) as hexosyl donor. Ample evidence has been presented that these enzymes are essential biosynthetic enzymes (Ballicora et al, 2003;Zeeman et al, 2010;Roach et al, 2012;Palm et al, 2013).…”