Putative receptor protein kinase. Could play a role in a differentiation signal. The CRINKLY4 (CR4) mutation affects leaf epidermis differentiation such that cell size and morphology are altered, and surface functions are compromised, allowing graft-like fusions between organs. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Homodimer. Also localized into protein export bodies. Belongs to the protein kinase superfamily. Ser/Thr protein kinase family.