Two expressed soybean genes with high sequence identity to tomato Pti1 kinase lack autophosphorylation activity

An important signaling pathway for disease resistance in tomato involves th e R gene product Pto which phosphorylates Pti1, a downstream member of this signaling cascade. Both Pto and Pti1 are Ser/Thr protein kinases capable o f autophosphorylation in vitro. Two soybean (Glycine max L, Merr, var. Hobb it) cDNAs (sPti1a and sPti1b) were cloned and sequenced and found to each h ave 78% amino acid sequence identity with tomato Pti1, Glutathione S-transf erase fusions of sPti1a and b expressed in Escherichia coil did not autopho sphorylate in vitro, but were efficiently phosphorylated by tomato Pto, Rep lacement of Tyr197 with an Asp that is invariant at this position in other protein kinases did not restore autophosphorylation activity to sPti1a or b , Tyr197 was also present in the Pti1 homologues of three distant relatives of G. max Together these results suggest that soybean Pti1 might function in a Pto-like signaling pathway that does not require Pti1 kinase activity. (C) 2000 Academic Press.