Major protein of resting rhizomes of Calystegia sepium (hedge bindweed) closely resembles plant RNases but has no enzymatic activity

The most abundant protein of resting rhizomes of Calystegia sepium (L.) R.B r. (hedge bindweed) has been isolated and its corresponding cDNA cloned. Th e native protein consists of a single polypeptide of 212 amino acid residue s and occurs as a mixture of glycosylated and unglycosylated isoforms. Both forms are derived from the same preproprotein containing a signal peptide and a C-terminal propeptide. Analysis of the deduced amino acid sequence in dicated that the C. sepium protein shows high sequence identity and structu ral similarity with plant RNases. However, no RNase activity could be detec ted in highly purified preparations of the protein. This apparent lack of a ctivity results most probably from the replacement of a conserved His resid ue, which is essential for the catalytic activity of plant RNases. Our find ings not only demonstrate the occurrence of a catalytically inactive varian t of an S-like RNase, but also provide further evidence that genes encoding storage proteins may have evolved from genes encoding enzymes or other bio logically active proteins.