Retrovirus infection is initiated by receptor-dependent fusion of the envelope to the cell membrane. The modular organization of the envelope protein of C type retroviruses has been exploited to investigate how binding of the surface subunit (SU) to receptor triggers fusion mediated by the transmembrane (TM) subunit. We show that deletion of the receptor-binding domain (RBD) from SU of Friend murine leukemia virus (Fr-MLV) abolishes infection that is restored by supplying RBD as a soluble protein. Infection by this mechanism remains dependent on receptor expression. When membrane attachment of the virus lacking RBD is reestablished by inserting the hormone erythropoietin, infection remains dependent on the RBD͞receptor complex. However, infection increases 50-fold to 5 ؋ 10 5 units͞ml on cells that also express the erythropoietin receptor. Soluble RBD from Fr-MLV also restores infection by amphotropic and xenotropic MLVs in which RBD is deleted. These experiments demonstrate that RBD has two functions: mediating virus attachment and activating the fusion mechanism. In addition, they indicate that receptor engagement triggers fusion by promoting a subgroup-independent functional interaction between RBD and the remainder of SU and͞or TM. R etrovirus infection is initiated by fusion of the virus envelope to the host cell membrane, resulting in delivery of the virion core into the cytoplasm. Fusion is triggered when the viral envelope proteins bind to receptors expressed on the host cell membrane. The retroviral envelope protein is a trimer of heterodimers formed between the surface (SU) and the transmembrane (TM) subunits (1, 2). At present, it is unclear how virus binding to receptor, which is mediated by SU, is coupled to fusion, which is mediated by TM. There is currently no evidence that the SU͞receptor complex participates in membrane fusion, nor that the receptor makes direct contact with TM. Therefore, critical intra-and possibly interenvelope interactions must occur that link receptor binding to the fusion mechanism.To investigate how this functional linking occurs, we have exploited the modular organization of the mammalian C-type retroviral envelope proteins (Fig. 1A). Sequence-based alignment of these proteins reveals a proline-rich region in the middle of SU that separates the N-terminal segment, which varies among murine leukemia virus (MLV) subgroups, from the C-terminal segment, which is relatively conserved (3-5). Chimeric envelope proteins, created by combining segments from viruses that use different receptors, are often functional and use the receptor specified by the N-terminal segment (4, 6, 7). In the SU of Friend MLV (Fr-MLV), the N-terminal 236 residues before the proline-rich region form a discrete domain (receptorbinding domain, RBD) that binds to receptor with high affinity and 1:1 stoichiometry (4,8,9). From functional studies informed by the atomic structure (Fig. 1B) (10), a pocket has been identified at the top of Fr-MLV RBD that is required for receptor binding and likely forms a cri...

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Aims/hypothesisThe DIRECT (Diabetes Research on Patient Stratification) Study is part of a European Union Framework 7 Innovative Medicines Initiative project, a joint undertaking between four industry and 21 academic partners throughout Europe. The Consortium aims to discover and validate biomarkers that: (1) predict the rate of glycaemic deterioration before and after type 2 diabetes onset; (2) predict the response to diabetes therapies; and (3) help stratify type 2 diabetes into clearly definable disease subclasses that can be treated more effectively than without stratification. This paper describes two new prospective cohort studies conducted as part of DIRECT.MethodsPrediabetic participants (target sample size 2,200–2,700) and patients with newly diagnosed type 2 diabetes (target sample size ~1,000) are undergoing detailed metabolic phenotyping at baseline and 18 months and 36 months later. Abdominal, pancreatic and liver fat is assessed using MRI. Insulin secretion and action are assessed using frequently sampled OGTTs in non-diabetic participants, and frequently sampled mixed-meal tolerance tests in patients with type 2 diabetes. Biosamples include venous blood, faeces, urine and nail clippings, which, among other biochemical analyses, will be characterised at genetic, transcriptomic, metabolomic, proteomic and metagenomic levels. Lifestyle is assessed using high-resolution triaxial accelerometry, 24 h diet record, and food habit questionnaires.Conclusions/interpretationDIRECT will yield an unprecedented array of biomaterials and data. This resource, available through managed access to scientists within and outside the Consortium, will facilitate the development of new treatments and therapeutic strategies for the prevention and management of type 2 diabetes.Electronic supplementary materialThe online version of this article (doi:10.1007/s00125-014-3216-x) contains peer-reviewed but unedited supplementary material, which is available to authorised users.