Phytosulfokine-α, a sulfated pentapeptide, stimulates the proliferation of rice cells by means of specific high- and low-affinity binding sites

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Abstract
Peptide growth factors were isolated from conditioned medium derived from rice ( Oryza sativa L.) suspension cultures and identified to be a sulfated pentapeptide [H-Tyr(SO 3 H)-Ile-Tyr(SO 3 H)-Thr-Gln-OH] and its C-terminal-truncated tetrapeptide [H-Tyr(SO 3 H)-Ile-Tyr(SO 3 H)-Thr-OH]. These structures were identical to the phytosulfokines originally found in asparagus ( Asparagus officinalis L.) mesophyll cultures. The pentapeptide [phytosulfokine-α (PSK-α)] very strongly stimulated colony formation of rice protoplasts at concentrations above 10 −8 M, indicating a similar mode of action in rice of phytosulfokines. Binding assays using 35 S-labeled PSK-α demonstrated the existence of both high- and low-affinity specific saturable binding sites on the surface of rice cells in suspension. Analysis of [ 35 S]PSK-α binding in differential centrifugation fractions suggested association of the binding with a plasma membrane-enriched fraction. The apparent K d values for [ 35 S]PSK-α binding were found to be 1 × 10 −9 M for the high-affinity type and 1 × 10 −7 M for the low-affinity type, with maximal numbers of binding sites of 1 × 10 4 sites per cell and 1 × 10 5 sites per cell, respectively. Competition studies with [ 35 S]PSK-α and several synthetic PSK-α analogs demonstrated that only peptides that possesses mitogenic activity can effectively displace the radioligand. These results suggest that a signal transduction pathway mediated by peptide factors is involved in plant cell proliferation.