One of the isoamylase isoforms, CMI294C, is required for semi-amylopectin synthesis in the rhodophyte .

Most rhodophytes synthesize semi-amylopectin as a storage polysaccharide, whereas some species in the most primitive class (Cyanidiophyceae) make glycogen. To know the roles of isoamylases in semi-amylopectin synthesis, we investigated the effects of gene ( and )-deficiencies on semi-amylopectin molecular structure and starch granule morphology in (Cyanidiophyceae). Semi-amylopectin content in a -disruption mutant () was not significantly different from that in the control strain, while that in a -disruption mutant () was much lower than those in the control strain, suggesting that CMI294C is essential for semi-amylopectin synthesis. Scanning electron microscopy showed that the strain contained smaller starch granules, while the strain had normal size, but donut-shaped granules, unlike those of the control strain. Although the chain length distribution of starch from the control strain displayed a semi-amylopectin pattern with a peak around degree of polymerization (DP) 11-13, differences in chain length profiles revealed that the strain has more short chains (DP of 3 and 4) than the control strain, while the strain has more long chains (DP ≥12). These findings suggest that CMI294C-type isoamylase, which can debranch a wide range of chains, probably plays an important role in semi-amylopectin synthesis unique in the Rhodophyta.