Aromatic Anchor at an Invariant Hormone-Receptor Interface. FUNCTION OF INSULIN RESIDUE B24 WITH APPLICATION TO PROTEIN DESIGN [Molecular Biophysics]

October 10th, 2014 by Pandyarajan, V., Smith, B. J., Phillips, N. B., Whittaker, L., Cox, G. P., Wickramasinghe, N., Menting, J. G., Wan, Z.-l., Whittaker, J., Ismail-Beigi, F., Lawrence, M. C., Weiss, M. A.

Crystallographic studies of insulin bound to fragments of the insulin receptor have recently defined the topography of the primary hormone-receptor interface. Here, we have investigated the role of PheB24, an invariant aromatic anchor at this interface and site of a human mutation causing diabetes mellitus. An extensive set of B24 substitutions has been constructed and tested for effects on receptor binding. Although aromaticity has long been considered a key requirement at this position, MetB24 was found to confer essentially native affinity and bioactivity. Molecular modeling suggests that this linear side chain can serve as an alternative hydrophobic anchor at the hormone-receptor interface. These findings motivated further substitution of PheB24 by cyclohexanylalanine (Cha), which contains a non-planar aliphatic ring. Contrary to expectations, ChaB24-insulin likewise exhibited high activity. Further, its resistance to fibrillation and rapid rate of hexamer disassembly-properties of potential therapeutic advantage-were enhanced. The crystal structure of the ChaB24 analog, determined as an R6 zinc-stabilized hexamer at a resolution of 1.5 Å, closely resembles that of wild-type insulin. The non-planar aliphatic ring exhibits two chair conformations with partial occupancies, each recapitulating the role of PheB24 at the dimer interface. Together, these studies have defined structural requirements of an anchor residue within the B24-binding pocket of the insulin receptor; similar molecular principles are likely to pertain to insulin-related growth factors. Our results highlight in particular the utility of non-aromatic side chains as probes of the B24 pocket and suggest that the non-standard Cha side chain may have therapeutic utility.
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