This work was supported by Australian National Health and Medical Research Council (NHMRC) Project grants 516729, 575539 and 1005896 and the Hazel and Pip Appel Fund (to M.C.L.), NHMRC Independent Research Institutes Infrastructure Support Scheme Grant 361646 and Victorian State Government Operational Infrastructure Support Grant (to the Walter and Eliza Hall Institute of Medical Research), NIH grant no. DK40949 (to M.A.W. and J.W.) and American Diabetes Association grantno. 1-11INI-31 (to J.W.), Grant Agency of the Czech Republic grant P207/11/P430 (to L.Z.), Research Project of the Academy of Sciences of the Czech Republic RVO:61388963 (to the Institute of Organic Chemistry and Biochemistry), NIH grants DK13914 and DK20595 (to D.F.S.), a BBSRC PhD studentship (to C.J.W.) and the UoY Research Priming Fund (to the York Structural Biology Laboratory). Part of this research was undertaken on the MX2 beamline at the Australian Synchrotron (AS), Victoria, Australia.We thank the DLS for access to beamline I24 and the Australian International Synchrotron Access Program for travel funds.