The carbohydrate binding type-21 or CBM21 domain is a 90-130 amino acid carbohydrate binding domain. The domain is named after proteins classified in carbohydrate-binding module (CBM) family 21 and is sometimes called starch-binding domain (SBD) [E1,1]. The CBM21 domain occurs in several eukaryotic proteins implicated in glycogen metabolism. A glucoamylase active site region (see <PDOC00646>) [E2] or α amylase catalytic domain [E3] can occur C-terminal to the CBM21 domain. The CBM21 domain of Rhizopus oryzae glucoamylase can bind to raw starch. Most conserved residues are located in a region with a length of 35 in the N-terminal part [2] and in a 15-25 residue motif II at the C-terminus of the domain [2,3,4].

Some proteins known to contain a CBM21 domain:

• Mammalian protein phosphatase 1 (PP1) regulatory subunit 3A, which seems to target PP1 to glycogen.
• Mammalian protein phosphatase 1 binding protein PTG, which binds the 3 key enzymes for the regulation of glycogen metabolism: phosphorylase kinase, phosphorylase A and glycogen synthase.
• Yeast protein phosphatase 1 regulatory subunits GAC1 and PIG1, which are regulators of glycogen synthase.
• Yeast GLC7-interacting protein 2 (GIP2), which interacts with the catalytic subunit (GLC7) of PP1.
• Yeast protein 2 interacting with GSY2 (PIG2), which interacts with glycogen synthase 2 (GSY2), a nutritionally regulated form.
• Rhizopus oryzae glucoamylase, which binds and hydrolyzes granular starch. R. oryzae is a fungus from e.g. decaying vegetables and it causes mucormycosis.

The profile we developed covers the entire CBM21 domain.