Rhamnogalacturonan lyase (RG-lyase) specifically recognizes and cleaves a-1,4 glycosidic bonds between L L -rhamnose and D D -galacturonic acids in the backbone of rhamno galacturonan-I, a major component of the plant cell wall polysaccharide, pectin. The three-dimensional structure of RGlyase from Aspergillus aculeatus has been determined to 1.5 A resolution representing the first known structure from polysaccharide lyase family 4 and of an enzyme with this catalytic specificity. The 508-amino acid polypeptide displays a unique arrangement of three distinct modular domains. Each domain shows structural homology to non-catalytic domains from other carbohydrate active enzymes.