RESPONSE OF MELON PLANTS TO SALT .3. MODULATION OF GTP-BINDING PROTEINS IN ROOT MEMBRANES

Antisera raised against peptide sequences homologous to the conserved region of the a subunit of heterotrimeric GTP-binding proteins (anti-G (alpha-common) antibody) from mammals and Dictyostelium discoideztm cr oss-reacted with proteins of microsomal and plasma membranes isolated from the roots of melon (Galia, Cucumis melo L.) seedlings. In microso mal membranes the immunolabeled sets of proteins were of approximately 66, 47, 37, 30 and 22 kD. The 22 kD protein band was absent in the pu rified plasmalemma. Immune-decoration of all protein bands was greatly reduced when excess purified transducin was included. The isolated mi crosomal membranes bound GTP gamma S with high affinity and specificit y. The K-d and the concentration of high affinity binding sites were s imilar to 10 nmol/L and similar to 5 pmol/mg total membrane protein, r espectively. The nucleotide order of potency was GTP gamma S-GTP>GDP m uch greater than ATP. Purified plasmalemma was relatively rich in high affinity GTP gamma S binding sites. Root membranes isolated from melo n seedlings grown in excess NaCl exhibited a marked increase in both t he concentration of GTP-binding sites and the amount of bound anti-G(a lpha-common) antibody. The number of binding sites per mg protein incr eased as a function of the salt concentration during growth. Moreover, with a salt sensitive cultivar of melon (Eshkolit Ha'Amaqim) the resp onse was apparent already at lower salinities. The findings identify G TP-binding proteins, presumably G proteins, in melon root membranes th at carry homologous sequence as well as comparable nucleotide specific ity and affinity to those of mammalian and simple eucaryotic heterotri meric G proteins. The modulation of these proteins by prolonged salini ty suggests a role for G proteins in the physiological response of pla nts to salination.