Nucleotide Sequence of a cDNA for Osmotin-Like Protein from Cultured Tobacco Cells

Abstract

Osmotin-like proteins, having amino terminal amino acid sequence homologous to osmotin, accumulate with osmotin to substantial levels in cultured tobacco (Nicotiana tabacum) cells (10). Osmotin and the analogous protein NP24 were first reported to accumulate in salt-adapted cells of tobacco and tomato (5, 8). However, recent studies indicated that the expression of osmotin and osmotin-like proteins are regulated developmentally in intact plants with high levels of expression in roots (4, 6, 9, 10). Furthermore, expression of these proteins was shown to be induced by tobacco mosaic virus infection and wounding (6, 10). Here, we report the nucleotide sequence of cDNA and its deduced amino acid sequence for an osmotin-like protein to learn more about the osmotin-like protein structure and its function in the stress response (Table I, Fig. 1). The amino acid sequence indicated that osmotin-like protein is synthesized as a 29 kD precursor and processed to the 26 kD mature polypeptide. Hydrophobicity analysis clearly shows the hydrophobic bias of the amino acids in the leader sequence. When the amino acid sequence of osmotin-like protein was compared to sequences of other proteins, osmotin-like protein was found to be quite similar to osmotin (9), salt-induced tomato protein (NP24, 5), the thaumatin-like pathogenesis-related protein of tobacco (PR-S, 1), maize aamylase/trypsin inhibitor (7), and thaumatin (3) (76, 77, 60, 54, and 47% identical, respectively). All of them were synthesized as premature polypeptide, which possessed highly hydrophobic signal peptides, and processed to mature form. The 16 cysteine residues involved in the disulfide bridges of thaumatin are perfectly conserved among the five proteins. These results indicated that these polypeptide genes were originated from a common ancestor gene. Sequence analysis also indicated several characteristics of amino acid sequence unique to osmotin-like protein; osmotin-like protein possessed a putative glycosylation site as well as PEST (proline-glutamate-serine-threonine rich) sequence, neither of which were found in osmotin or tobacco pathogenesis-related protein (PR-S). These findings suggest that osmotin-like protein may have a function different from other proteins with similar sequences. Sequence comparison revealed that osmotin-like protein was highly homologous to one of the anti-virus proteins recently isolated from the same variety of tobacco (2). High homology suggested that osmotinlike protein might be a precursor of anti-virus protein gp22. Isolated cDNA of osmotin-like protein provides an opportunity to examine whether osmotin-like protein has anti-viral activity or not.