120- and 160-kDa Receptors for Endogenous Mitogenic Peptide, Phytosulfokine-α, in Rice Plasma Membranes*

Plant cells in culture secrete a sulfated peptide named phytosulfokine-α (PSK-α), and this peptide induces the cell division and/or cell differentiation by means of specific high and low affinity receptors. Putative receptor proteins for this autocrine type growth factor were identified by photoaffinity labeling of plasma membrane fractions derived from rice suspension cells. Incubation of membranes with a photoactivable 125I-labeled PSK-α analog, [N ε-(4-azidosalicyl)Lys5]PSK-α (AS-PSK-α), followed by UV irradiation resulted in specific labeling of 120- and 160-kDa bands in SDS-polyacrylamide gel electrophoresis. The labeling of both bands was completely inhibited by unlabeled PSK-α and partially decreased by PSK-α analogs possessing moderate binding activities. In contrast, PSK-α analogs that have no biological activity showed no competition for125I-AS-PSK-α binding, confirming the specificity of binding proteins. Analysis of the affinity of 125I incorporation into the protein by ligand saturation experiments gave apparent K d values of 5.0 nm for the 120-kDa band and 5.4 nm for the 160-kDa band, suggesting that both proteins correspond to the high affinity binding site. Treatment of 125I-AS-PSK-α cross-linked proteins with peptide N-glycosidase F demonstrated that both proteins contained approximately 10 kDa of N-linked oligosaccharides. Specific cross-linking of 125I-AS-PSK-α was also observed by using plasma membranes derived from carrot and tobacco cells, indicating the widespread occurrence of the binding proteins. Together, these data suggest that the 120- and 160-kDa proteins are PSK-α receptors that mediate the biological activities of PSK-α.