A 2.7 angstrom resolution x-ray diffraction analysis of a trypsin-modified form of the Escherichia coli elongation factor Tu reveals that the GDP-binding domain has a structure similar to that of other nucleotide-binding proteins. The GDP ligand is located at the COOH-terminal end of the beta sheet and is linked to the protein via a Mg2+ ion salt bridge. The location of the guanine ring is unusual; the purine ring is located on the outer edge of the domain, not deep within a hydrophobic pocket. The amino acids from Pro10 to Arg44 and from Gly59 to Glu190 have been assigned to the electron density with computer graphic techniques, and the resulting model is consistent with all known biochemical data. An analysis of the structure reveals that four regions of the amino acid sequence that are homologous with the family of ras oncogene proteins, termed p21, are located in the vicinity of the GDP-binding site, and most of the invariant amino acids shared by the proteins interact directly with the GDP ligand.

Pectate lyases are secreted by pathogens and initiate soft-rot diseases in plants by cleaving polygalacturonate, a major component of the plant cell wall. The three-dimensional structure of pectate lyase C from Erwinia chrysanthemi has been solved and refined to a resolution of 2.2 angstroms. The enzyme folds into a unique motif of parallel beta strands coiled into a large helix. Within the core, the amino acids form linear stacks and include a novel asparagine ladder. The sequence similarities that pectate lyases share with pectin lyases, pollen and style proteins, and tubulins suggest that the parallel beta helix motif may occur in a broad spectrum of proteins.

The structure of EF-Tu-GDP is nearly identical to that of a trypsin-modified form of EF-Tu-GDP, demonstrating conclusively that the protease treatment had not altered any essential structural features. The present structure represents the first view of an ordered Switch I region in EF-Tu-GDP and reveals similarities with two other GTPases complexed with GDP: Ran and ADP-ribosylation factor-1. A comparison of the Switch I regions of the GTP and GDP forms of EF-Tu also reveals that a segment, six amino acids in length, completely converts from an alpha helix in the GTP complex to beta secondary structure in the GDP form. The alpha to beta switch in EF-Tu may represent a prototypical activation mechanism for other protein families.

The structure and function of Erwinia chrysanthemi pectate lysase C, a plant virulence factor, is reviewed to illustrate one mechanism of pathogenesis at the molecular level. Current investigative topics are discussed in this paper. P lant cell walls are primarily polysaccharide in composition. A simple but major pathogenic mechanism in plants involves degradation of the cell wall by a battery of polysaccharidases secreted by pathogens. Most of the degradative enzymes are glycoside hydrolases, which degrade the cellulose and pectate matrices by the addition of water to break the glycosidic bonds. The pectate network is also degraded by polysaccharide lyases, which cleave the glycosidic bonds via a ␤-elimination mechanism. To better understand the latter virulence mechanism, research has been carried out on pectate lyase C, a pectolytic enzyme secreted by the pathogenic bacterium Erwinia chrysanthemi. The story of pectate lyase C illustrates how structural techniques have contributed to a detailed understanding of polysaccharide recognition and the lyase cleavage mechanism. In the process, a novel protein structural fold and a unique catalytic role for an arginine have been discovered. The structural results have also provided the first atomic description of a pectate fragment, which differs considerably from the popular view in conformation as well as the mode of interactions with Ca 2ϩ ions. Finally, the growing structural database of pectolytic enzymes is enabling researchers to elucidate subtle structural differences that are responsible for the specific recognition of a unique oligosaccharide sequence from a heterogeneous mix in the plant cell wall. Such knowledge will ultimately lead to a better understanding of the characteristics that render the host susceptible to attack by a particular pathogen.