AMINO-ACID-SEQUENCE OF AN EXTRACELLULAR, PHOSPHATE-STARVATION-INDUCED RIBONUCLEASE FROM CULTURED TOMATO (LYCOPERSICON-ESCULENTUM) CELLS

The primary structure of an extracellular ribonuclease (RNase LE) from P(i)-depleted media of cultured cells of Lycopersicon esculentum L. cv. Lukullus has been determined. This was carried out by analysis of peptides isolated after enzymatic and chemical cleavage of the reduced and S-ethylpyridylated protein. RNase LE consists of 205 amino acid residues and has a molecular mass of 22666 Da and an isoelectric point of 4.24. The enzyme contains 10 half-cystines. There are no potential N-glycosylation sites in the sequence. The sequence of RNase LE is homologous with those of self-incompatibility proteins of several higher plant species and with those of a number of fungal RNases. The sequence similarity with the family of self-incompatibility proteins is greater than with the fungal RNases, suggesting that the self-incompatibility proteins arose from ancestral RNase by gene duplication after the divergence of higher plants and fungi. Two pentapeptide sequences, i.e. HGLWP and KHGTC (or KHGSC), are present at identical positions in all the aligned proteins, suggesting that they contribute to the active site.