Amino acid sequence of an intracellular, phosphate-starvation-induced ribonuclease from cultured tomato (Lycopersicon esculentum) cells.

The primary structure of an intracellular ribonuclease (RNase LX) from cultured tomato (Lycopersicon esculentum) cells has been determined. Previous studies have shown that the protein is located inside the tomato cells but outside the vacuoles and that its synthesis is induced after depleting the cells for phosphate [Löffler, A., Abel, S., Jost, W., Beintema, J. J., Glund, K. (1992) Plant Physiol. 98, 1472-1478]. Sequence analysis was carried out by analysis of peptides isolated after enzymatic and chemical cleavage of the protein. RNase LX consists of 213 amino acids and has a molecular mass of 24300 Da and an isoelectric point of 5.33. The enzyme contains 10 half-cystines and there are no potential N-glycosylation sites detectable in the sequence. RNase LX, as compared to an extracellular tomato RNase (RNase LE), which is also phosphate regulated and the amino acid sequence of which was recently established [Jost, W., Bak, H., Glund, K., Terpstra, P. & Beintema, J. J. (1991) Eur. J. Biochem. 198, 1-6] has 60% of all amino acids identical and in identical positions, revealing a high degree of similarity between both proteins. In contrast to RNase LE, RNase LX has a C-terminal extension of nine amino acids. The C-terminal tetrapeptide HDEF may be a retention signal of the protein in the endoplasmic reticulum.