AmyA contributes to the glycogen synthesis in Sulfolobus acidocaldarius

Glycogen, an α-1,4 linked glucose polymer with α-1,6 linked branches, accumulates in Sulfolobus acidocaldarius in granular form and contributes to stress resistance. While the glg operon responsible for glycogen metabolism has been studied, the gene responsible for branch formation remained elusive. Interestingly, the ΔAmyA mutant failed to accumulate glycogen. We hypothesized that amyA is responsible for branch formation in glycogen. In this study, AmyA was characterized to have dual activities as an α-amylase and a glycogen-branching enzyme. Glycogen extracted from S. acidocaldarius exhibited α-1,6 linked glucose branches, with most branches containing 5-13 glucose units. AmyA showed a preference for synthesizing branches with a degree of polymerization of 6. Structural modeling of AmyA, in comparison with GH57 glycogen-branching enzymes (GBE), revealed the presence of key amino acids essential for branching activity, located in positions structurally analogous to those in GH57 GBEs, enabling AmyA to function as a glycogen-branching enzyme. Alignment of the glg operons showed that amyA is conserved, while glgB is absent in most Crenarchaeota. Based on these findings, we propose that AmyA synthesizes α-1,6 branches in glycogen, substituting for the role of GlgB in Crenarchaeota.