Antibodies have been widely used for cancer therapy owing to their ability to distinguish cancer cells by recognizing cancer-specific antigens. Epidermal growth factor receptor (EGFR) is a promising target for the cancer therapeutics, against which several antibody clones have been developed and brought into therapeutic use. Another antibody clone, 528, is an antagonistic anti-EGFR antibody, which has been the focus of our antibody engineering studies to develop cancer drugs. In this study, we explored the interaction of 528 with the extracellular region of EGFR (sEGFR) via binding analyses and structural studies. Dot blotting experiments with heat treated sEGFR and surface plasmon resonance binding experiments revealed that 528 recognizes the tertiary structure of sEGFR and exhibits competitive binding to sEGFR with EGF and cetuximab. Single particle analysis of the sEGFR–528 Fab complex via electron microscopy clearly showed the binding of 528 to domain III of sEGFR, the domain to which EGF and cetuximab bind, explaining its antagonistic activity. Comparison between the two-dimensional class average and the cetuximab/sEGFR crystal structure revealed that 528 binds to a site that is shifted from, rather than identical to, the cetuximab epitope, and may exclude known drug-resistant EGFR mutations.

Landomycin A (LaA) is the largest member of the landomycin group of angucyclic polyketides. Its unusual structure and strong anticancer properties have attracted great interest from chemists and biologists alike. This, in particular, has led to a detailed picture of LaA biosynthesis in Streptomyces cyanogenus S136, the only known LaA producer. LanK is a TetR family repressor protein that limits the export of landomycins from S136 cells until significant amounts of the final product, LaA, have accumulated. Landomycins carrying three or more carbohydrate units in their glycosidic chain are effector molecules for LanK. Yet, the exact mechanism that LanK uses to distinguish the final product, LaA, from intermediate landomycins and sense accumulation of LaA was not known. Here, we report crystal structures of LanK, alone and in complex with LaA, and bioassays of LanK's interaction with synthetic carbohydrate chains of LaA (hexasaccharide) and LaE (trisaccharide). Our data collectively suggest that the carbohydrate moieties are the sole determinants of the interaction of the landomycins with LanK, triggering the latter's dissociation from the lanK-lanJ intergenic region via structure conversion of the helices in the C-terminal ligand-binding domain. Analysis of the available literature suggests that LanK represents an unprecedented type of TetR family repressor that recognises the carbohydrate portion of a natural product, and not an aglycon, as it is the case, for example, with the SimR repressor involved in simocyclinone biosynthesis.

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