Involved in the negative regulation of the K(+) potassium channel AKT1 by its dephosphorylation, antagonistically to CIPK proteins (e.g. CIPK23) (PubMed:17898163). Functions as positive regulator of abscisic acid-mediated cell signaling during seedling growth (PubMed:22404835). Involved in the regulation of seed dormancy (PubMed:23378449). Acts as negative regulator of seed dormancy by inhibiting abscisic signaling and subsequently activating gibberellic acid signaling (PubMed:23378449). H2O + O-phospho-L-seryl-[protein] = L-seryl-[protein] + phosphate H2O + O-phospho-L-threonyl-[protein] = L-threonyl-[protein] + phosphate Binds 2 magnesium or manganese ions per subunit. Part of a K(+)-channel calcium-sensing kinase/phosphatase complex composed by a calcium sensor CBL (CBL1, CBL2, CBL3 or CBL9), a kinase CIPK (CIPK6, CIPK16 or CIPK23), a phosphatase PP2C (AIP1) and a K(+)-channel (AKT1) (PubMed:17898163). Interacts with AKT1 and CIPK23 (PubMed:17898163). Interacts with PYL8/RCAR3 in an abscisic acid-independent (PubMed:22404835). Interacts with PYR1/RCAR11 in an abscisic acid-dependent manner (PubMed:23378449). Probably associated to the plasma membrane when interacting with AKT1 and CIPK23 (Probable). Localizes to nucleus when interacting with PYL8/RCAR3 (PubMed:22404835). Expressed in shoot meristem, vascular tissues of cotyledons, and in primary roots surrounding the root meristem (PubMed:22404835). Highly expressed in seeds (PubMed:23378449). Induced by abscisic acid (ABA). No visible phenotype under normal growth conditions, but mutant plants exhibit reduced sensitivity to abscisic acid (ABA) and glucose during seed germination and seedling stage (PubMed:22404835). Freshly harvested seeds of mutant plants exhibit increased seed dormancy (PubMed:23378449). Honsu means abnormal drowsiness in Korean. Belongs to the PP2C family.