Characterization of specific binding sites for a mitogenic sulfated peptide, phytosulfokine-α, in the plasma-membrane fraction derived from Oryza sativa L.

Treatment of rice cells with an endogenous mitogenic peptide, phytosulfokine-alpha (PSK-alpha), results in cell proliferation. In the present study, [H-3]PSK-alpha prepared by catalytic reduction of a PSK-alpha. analog containing tetradehydroisoleucine was employed to identify putative PSK-alpha target molecules on rice plasma membranes. Membrane binding of the ligand was found to be saturable, reversible and pH dependent. Scatchard analysis demonstrated the existence of both high- and low-affinity binding sites with K-d values of 1.4 nM and 27 nM, respectively. Competition studies with [H-3]PSK-alpha and several PSK-alpha analogs showed that displacing activity closely corresponds to the ability to induce cell proliferation. The properties of the binding sites distributed on plasma membranes are consistent with the function of PSK-alpha receptors in activating a cascade of molecular events involved in plant cell proliferation.