STRUCTURAL-ANALYSIS AND MOLECULAR-MODEL OF A SELF-INCOMPATIBILITY RNASE FROM WILD TOMATO

Self-incompatibility RNases (S-RNases) are an allelic series of style glycoproteins associated with rejection of self-pollen in solanaceous plants. The nucleotide sequences of S-RNase alleles from several gener a have been determined, but the structure of the gene products has onl y been described for those from Nicotiana alata. We report on the N-gl ycan structures and the disulfide bonding of the S-3-RNase from wild t omato (Lycopersicon peruvianum) and use this and other information to construct a model of this molecule. The S-3-RNase has a single N-glyco sylation site (Asn-28) to which one of three N-glycans is attached. S- 3-RNase has seven Cys residues; six are involved in disulfide linkages (Cys-16-Cys-21, Cys-46-Cys-91, and Cys-166-Cys-177), and one has a fr ee thiol group (Cys-150). The disulfide-bonding pattern is consistent with that observed in RNase Rh, a related RNase for which radiographic -crystallographic information is available. A molecular model of the S -3-RNase shows that four of the most variable regions of the S-RNases are clustered on one surface of the molecule. This is discussed in the context of recent experiments that set out to determine the regions o f the S-RNase important for recognition during the self-incompatibilit y response.