Xyloglucan O-acetyltransferase 1 (XGOAT1, also known as protein altered xyloglucan 4, AXY4) predominantly catalyze 6-O-monoacetylation of Gal residues in the Fuc-Gal-Xyl trisaccharide side chains of xyloglucan oligomers []. Loss of AXY4 transcript results in a complete lack of O-acetyl substituents on xyloglucan in several tissues, except seeds, while the O-acetylation level of other polysaccharides is not affected [].
GAL11/MED15 (mediator of RNA polymerase II transcription subunit 15) acts in the general regulation of GAL structural genes and is required for full expression for several genes in this pathway, including GALs 1,7, and 10 in Saccharomyces cerevisiae []. GAL11 function is dependent on GCN4 functionality and binds GCN4 in a degenerate manner with multiple orientations found at the GCN4-GAL11 interface [, , ]. This entry represents an activator-binding domain of GAL11. The tertiary structure of this domain has been solved and shows a four-helix fold with a small shallow hydrophobic cleft at its centre into which three Gcn4 aromatic/aliphatic residues are inserted [].
The lacI-type HTH domain is a DNA-binding, helix-turn-helix (HTH) domain ofabout 50-60 residues present in the lacI/galR family of transcriptionalregulators involved in metabolic regulation in prokaryotes. Most of thesebacterial regulators recognize sugar-inducers. The family is named after theEscherichia coli lactose operon repressor lacI and galactose operon repressorgalR. LacI-type regulators are present in diverse bacterial genera, in thecytoplasm. The 'helix-turn-helix' DNA-binding motif is located in theN-terminal extremity of these transcriptional regulators. The C-terminal partof lacI-type regulators contains several regions that can be involved in (1)binding of inducers, which are sugars and their analogues and (2)oligomerization. The lac repressor is a tetramer, whilst the gal and cytrepressors are dimers. LacI-type transcriptional regulators are important inthe coordination of catabolic, metabolic and transport operons [, ].Several structures of lacI-type transcriptional regulators have been resolvedand their DNA-binding domain encompasses a headpiece, formed by a fold ofthree helices, followed by a hinge region, which can form a fourth α-helixor hinge-helix. The helix-turn-helix motif comprises thefirst and second helices, the second being called the recognition helix. TheHTH is involved in DNA-binding into the major groove, while the hinge-helixfits into the minor groove and the complete domain specifically recognizes theoperator DNA [].Some proteins known to contain a lacI-type HTH domain:Bacillus subtilis ccpA and ccpB, transcriptional regulators involved inthe catabolic repression of several operons.Salmonella typhimurium fruR, the fructose repressor, involved in theregulation of a large number of operons encoding enzymes which take part incentral pathways of carbon metabolism.Escherichia coli lacI, the lactose operon repressor, serving as a model forgene regulation.Escherichia coli purF and purR, repressors involved in the regulation ofenzymes for purine nucleotide synthesis.Haemophilus influenzae galR, a repressor of the galactose operon.
The PapD-like superfamily of periplasmic chaperones directs the assembly of over 30 diverse adhesive surface organelles that mediate the attachment of many different pathogenic bacteria to host tissues, a critical early step in the development of disease. PapD, the prototypical chaperone, is necessary for the assembly of P pili. P pili contain the adhesin PapG, which mediates the attachment of uropathogenic Escherichia coli to Gal(alpha) Gal receptors present on kidney cells and are critical for the initiation of pyelonephritis. The PapD-like chaperones consist of two Ig-like domains oriented toward each other, forming L-shaped molecules. In the chaperone-subunit complex, the G1beta strand of the chaperone completes an atypical Ig fold in the subunit by occupying the groove and running parallel to the subunit C-terminal F strand. This donor strand complementation interaction simultaneously stabilises pilus subunits and caps their interactive surfaces, preventing their premature oligomerisation in the periplasm. During pilus biogenesis, the highly conserved N-terminal extension of one subunit has been proposed to displace the chaperone G1beta strand from its neighbouring subunit in a mechanism termed donor strand exchange [].This entry represents the immunoglobulin (Ig)-like β-sandwich domain found in PapD, as well as in other periplasmic chaperone proteins that include FimC and SfaE from E. coli, and Caf1m from Yersinia pestis []. In addition, major sperm proteins (MSP) and other related sperm proteins (such as WR4 and SSP-19) contain an Ig-like domain with a similar structural fold to PapD [, ]. Major sperm proteins are central components in molecular interactions underlying sperm motility, with many isoforms existing in Caenorhabditis elegans.
