olute tương tác trong tổ hợp giống như fasciclinAs discussed above, the prevalence of soluble ligands such as sugars, amino acids, and metabolic intermediates for different sequence superfamilies of the clade suggested an ancestral solute-binding role for these proteins. Analysis of the interactions with respect to the shared structural core of this assemblage suggests that the insert and the lateral shelf form an interface for soluble ligand interaction in fasciclin, GS-N, and phosphoribosyl-AMP cyclohydrolase domains [59,77,109]. Furthermore, in glutamine synthetase this interaction might indirectly contribute to catalysis via a conserved aspartate from this region that interacts with the substrate bound at the active site and helps in anchoring it there. This suggests that ancestral versions of this assemblage probably mediated a generic ligand interaction via a similar interface. The interactions of the L25 domain via this interface, if any, remain unknown. However, it is known to contact 5S rRNA via the exposed face [78]. FimD, which appears to be a distant relative of the fasciclin-like assemblage, assumes a classical barrel configuration, with the "open-end" of the barrel providing an interface for interacting with the FimC immunoglobulin domain [61]. Similar "open-ends" of topologically unrelated barrels like the OB fold, PRC, and SH3 barrels are known to mediate interactions with ligands in a like manner [110-112]. The loop between the penultimate two strands of the core FimD β-GF domain is one of the major determinants of the interaction and this feature is comparable to certain interactions of the phosphoribosyl-AMP cyclohydrolase domain (see below).
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