Essential determinant for high-affinity spermidine transport. Required for the activation of the plasma membrane proton pump PMA1 via phosphorylation of 'Ser-899'. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Present with 1420 molecules/cell in log phase SD medium. Belongs to the protein kinase superfamily. Ser/Thr protein kinase family.