MOLECULAR CHARACTERIZATION OF A CDNA SEQUENCE ENCODING THE BACKBONE OF A STYLE-SPECIFIC 120 KDA GLYCOPROTEIN WHICH HAS FEATURES OF BOTH EXTENSINS AND ARABINOGALACTAN PROTEINS

Nicotiana alata has a style-specific hydroxyproline-rich glycoprotein (the 120 kDa glycoprotein) which has properties of both extensins and AGPs [19, 20]. The 120 kDa glycoprotein is a soluble component in the extracellular matrix of the transmitting tract of styles where it acco unts for ca. 9% of the total buffer-soluble protein. Here we describe the molecular cloning of a cDNA representing the gene NaPRP5 which enc odes the backbone of the 120 kDa glycoprotein. Expression of mRNA is r estricted to styles, consistent with observations on the distribution of the 120 kDa glycoprotein. Levels of accumulation of the transcript encoding the 120 kDa protein backbone are not altered significantly by pollination with either compatible or incompatible pollen. The protei n backbone of the 120 kDa glycoprotein, as predicted by the cDNA seque nce, is composed of three distinct domains. The sequence of these doma ins, together with linkage analysis of the carbohydrate component of t he 120 kDa glycoprotein, allows predictions of the likely distribution of substituent glycosyl chains along the protein backbone. The simila rity of the C-terminal domains of the 120 kDa glycoprotein and GaRSGP, the galactose-rich style glycoprotein of N. alara, is consistent with the two molecules sharing a common antigenic domain in their backbone s [31]. The sharing of domains between distinct hydroxyproline-rich gl ycoproteins suggests that identification of a glycoprotein of this cla ss solely by its protein or carbohydrate epitope is not valid.