Response of melon plants to salt: 3. Modulation of GTP-binding proteins in root membranes

Abstract

Summary Antisera raised against peptide sequences homologous to the conserved region of the α subunit of hetero-trimeric GTP-binding proteins (anti-G α.common antibody) from mammals and Dictyostelium discoideum cross-reacted with proteins of microsomal and plasma membranes isolated from the roots of melon (Galia, Cucumis melo L.) seedlings. In microsomal 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 purified plasmalemma. Immuno-decoration of all protein bands was greatly reduced when excess purified transducin was included. The isolated microsomal membranes bound GTPγS with high affinity and specificity. The K d and the concentration of high affinity binding sites were ∼10 nmol/L and ∼5 pmol/mg total membrane protein, respectively. The nucleotide order of potency was GTPγS∼GTP>GDP≫ATP. Purified plasmalemma was relatively rich in high affinity GTPγS binding sites. Root membranes isolated from melon seedlings grown in excess NaCl exhibited a marked increase in both the concentration of GTP-binding sites and the amount of bound anti-G α-common antibody. The number of binding sites per mg protein increased as a function of the salt concentration during growth. Moreover, with a salt sensitive cultivar of melon (Eshkolit Ha’Amaqim) the response was apparent already at lower salinities. The findings identify GTP-binding proteins, presumably G proteins, in melon root membranes that carry homologous sequence as well as comparable nucleotide specificity and affinity to those of mammalian and simple eucaryotic heterotrimeric G proteins. The modulation of these proteins by prolonged salinity suggests a role for G proteins in the physiological response of plants to salination.