PARTIAL CHARACTERIZATION and LIGHT‐INDUCED REGULATION OF GTP‐BINDING PROTEINS IN Lemna paucicostata

Abstract

Binding of GTP‐binding proteins with [35S]GTP7S in the extract containing membrane components of Lemna paucicostata 441 was inhibited by red or far red light by 20 to 25%, but blue light showed no or little effect. The plant used for the preparation of the extract was subjected to single darkness for 8 h, as both red and far red light inhibit flowering. The extract treated with 1% Lubrol was fractionated by gel filtration. Four species of GTP‐binding proteins, GL1, GL2, GL3 and GL4 were detected with Km values 3, 7, 80 and 4 nM, respectively. GL1, GL2 and GL3 were ADP‐ribosylated by pertussis toxin. The extract activated by [35S]GTP‐γS in darkness, under red light or under far red light was treated with 1% Lubrol and subsequent gel filtration of the extracts made it possible to detect GTP‐binding protein with a small molecular weight only in an extract labeled in darkness. The reduction in the molecular weight of GTP‐binding protein from the larger molecule associated with the binding of [35S]GTPγS was confirmed by rechromatography of the larger molecule activated by [35S]GTPγS in darkness. The binding of GL2 and/or GL3 with [35S]GTPγS was suggested to be inhibited by red or far red light.

The Role(s) of Calcium Ions in Phytochrome Action

“…The presence of a protein which binds ["SIGTPy-S has been found in many plant systems (Hasunuma et al, 1987;Blum et al, 1988;Drcbbak et al, 1988;Korolkov et al, 1990;Romero et al, 1991b). These proteins were ADP-ribosylated in the presence of cholera toxin (Romero et al, 1991a) and they possess GTPase activity (Korolkov et al, 1990) and were recognized by antibodies raised against a highly conserved peptide of most G, asubunits (Blum et al, 1988;Korolkov et al, 1990;Romero et al, 1991b).…”

“…These proteins were ADP-ribosylated in the presence of cholera toxin (Romero et al, 1991a) and they possess GTPase activity (Korolkov et al, 1990) and were recognized by antibodies raised against a highly conserved peptide of most G, asubunits (Blum et al, 1988;Korolkov et al, 1990;Romero et al, 1991b). Furthermore, [35]GTP-y-Sbinding proteins have been isolated from membrane fractions (Hasunuma et al, 1987;Blum et al, 1988;Drcbbak et al , 1988;Korolkov et al, 1990). Despite the fact that G-proteins isolated from plants may differ in molecular mass they have similar biochemical and molecular properties to G, isolated from animal cells (Stryer and Bourne, 1986;Gilman, 1987).…”

“…However, an activator of G-proteins, GTP-y-S, induced swelling of the protoplasts to the same extent as after R, both in darkness and after a control FR irradiation as long as essential Ca2+ was present (Bossen et a[., 1990). Hasunuma et al (1987) were the first to report binding of [35S]GTP-y-S to proteins in extracts of Lemna paucicostata 441 containing membrane components. They found that R and FR inhibited GTPbinding to these proteins and proposed that phytochrome (Pfr) may function in the same way as the photoexcited state of rhodopsin in animals.…”

A membrane-delimited pathway of G-protein regulation of the guard-cell inward K+ channel.

“…There is a growing body of evidence supporting the existence of G proteins in plants ( [10][11][12][13][14]. There is also some evidence showing the involvement of G proteins in signal transduction in plant cells (10,(13)(14)(15)(16).…”

“…There is also some evidence showing the involvement of G proteins in signal transduction in plant cells (10,(13)(14)(15)(16). We have previously reported evidence that whole-cell inward K+ currents of guard cells (17) and whole-cell outward K+ currents of mesophyll cells (18) are regulated by G proteins.…”