Regulates the function of the receptor protein kinase WAK1. Interacts with WAK1 (via the extracellular domain). Component of a 500 kDa complex, composed of GRP3 or GRP3-S, WAK1 and KAPP. A number of isoforms are produced. According to EST sequences. 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.