DISULFIDE BONDING IN A STYLAR SELF-INCOMPATIBILITY RIBONUCLEASE OF NICOTIANA-ALATA

Many flowering plants have developed a self-incompatibility mechanism, which is controlled by a single polyallelic locus (the S-locus), to p revent inbreeding. The products of the S-locus in the styles of solana ceous plants are an allelic series of glycoproteins with RNase activit y [McClure, B. A., Hating, V., Ebert, P. R., Anderson, M. A., Simpson, R. J., Sakiyama, F. & Clarke, A. E. (1989) Nature 342, 955-957]. Thes e S-RNases show some amino-acid-sequence similarity with two fungal RN ases (T-2 and Rh), including the presence of two active-site His resid ues, which suggests a common three-dimensional structure. Disulphide b onding is important in the maintenance of the three-dimensional struct ure of the fungal RNases [Kurihara, H., Mitsui, Y., Ohgi, K., Irie, M. , Mizuno, H. & Nakamura, T. (1992) FEES Lett. 306, 189-192] and the S- RNases [Tsai, D. S., Lee, H.-S., Post, L. C., Kreiling, K. M. & Kao, T .-H. (1992) Sex. Plant Reprod. 5, 256-263]. We have used the S-2-allel e RNase of Nicotiana alata, which has nine Cys residues, to establish the pattern of disulphide bonding. The disulphide bonds Cys16-Cys21, C ys45-Cys94, Cys153-Cys182 and Cys165-Cys176 are consistent with the S- 2-RNase having a similar three-dimensional structure to RNase Rh. A fr ee Cys residue (Cys95) adjacent to Cys45-Cys94 promotes a rapid specif ic disulphide migration when the protein is exposed to denaturing cond itions.