MOLECULAR-CLONING OF CDNAS ENCODING THE PROTEIN BACKBONES OF ARABINOGALACTAN-PROTEINS FROM THE FILTRATE OF SUSPENSION-CULTURED CELLS OF PYRUS-COMMUNIS AND NICOTIANA-ALATA

This paper reports the isolation of cDNAs encoding the protein backbon e of two arabinogalactan-proteins (AGPs), one from pear cell suspensio n cultures (AGP Pc2) and the other from suspension cultures of Nicotia na alata (AGP Na2). The proteins encoded by these cDNAs are quite diff erent from the 'classical' AGP backbones described previously for AGPs isolated from pear suspension cultures and extracts of N. alata style s. The cDNA for AGP Pc2 encodes a 294 amino acid protein, of which a r elatively short stretch (35 amino acids) is Hyp/Pro rich; this stretch is flanked by sequences which are dominated by Asn residues. Asn resi dues are not a feature of the 'classical' AGP backbones in which Hyp/P ro, Ser, Ala and Thr account for most of the amino acids. The cDNA for AGP Na2 encodes a 437 amino acid protein, which contains two distinct domains: one rich in Hyp/Pro, Ser, Ala, Thr and the other rich in Asn , Tyr and Ser. The composition and sequence of the Pro-rich domain res embles that of the 'classical' AGP backbone. The Asn-rich domains of t he two cDNAs described have no sequence similarity; in both cases they are predicted to be processed to give a mature backbone with a compos ition similar to that of the 'classical' AGPs. The study shows that di fferent AGPs can differ in the amino acid sequence in the protein back bone, as well as the composition and sequence of the arabinogalactan s ide-chains. It also shows that differential expression of genes encodi ng AGP protein backbones, as well as differential glycosylation, can c ontribute to the tissue specificity of AGPs.