Novel, starch-like polysaccharides are synthesized by an unbound form of granule-bound starch synthase in glycogen-accumulating mutants of Chlamydomonas reinhardtii

In vascular plants, mutations leading to a defect in debranching enzyme lea d to the simultaneous synthesis of glycogen-like material and normal starch . In Chlamydomonas reinhardtii comparable defects lead to the replacement o f starch by phytoglycogen. Therefore, debranching was proposed to define a mandatory step for starch biosynthesis. We now report the characterization of small amounts of an insoluble, amylose-like material found in the mutant algae. This novel, starch-like material was shown to be entirely dependent on the presence of granule-bound starch synthase (GBSSI), the enzyme respo nsible for amylose synthesis in plants. However, enzyme activity assays, so lubilization of proteins from the granule, and western blots all railed to detect GBSSI within the insoluble polysaccharide matrix. The glycogen-like polysaccharides produced in the absence of GBSSI were proved to be qualitat ively and quantitatively identical to those produced in its presence. There fore, we propose that GBSSI requires the presence of crystalline amylopecti n for granule binding and that the synthesis of amylose-like material can p roceed at low levels without the binding of GBSSI to the polysaccharide mat rix. Our results confirm that amylopectin synthesis is completely blocked i n debranching-enzyme-defective mutants of C. reinhardtii.