Regulates the function of the receptor protein kinase WAK1, and namely the phosphorylation of OEE2. Interacts (via Cys-rich C-terminus) with WAK1 (via the extracellular domain). Component of a 500 kDa complex, composed of GRP3 or GRP3-S, WAK1 and KAPP. Predominantly expressed in leaves and stems. By salicylic acid. Transient induction by drought. Up-regulated by itself. The Cys-rich C-terminus (111-145) is essential for the interaction with WAK1. GRP3 and GRP3S bind to WAK1, WAK3 and WAK5, but GRP2, GRP4,GRP6, GRP7 and GRP8 did not bind to any of the WAK isoforms. Belongs to the GRP family.