Options for the stabilization of well-defined helical peptide drugs and basic research tools have received considerable attention in the last decade. attachment to a target cell HIV entry is effected by fusion of the respective cell membranes mediated by gp41. In the prefusogenic state gp41 Coumarin 7 is trimerized via an N-terminal heptad repeat (NHR) and an N-terminal fusion peptide inserts into the target membrane. The protein then undergoes a conformational rearrangement in which the C-peptide packs as Coumarin 7 an antiparallel helix against the surface of the NHR trimer. Anchoring of the respective protein termini into viral and cell membranes ensures that this rearrangement requires membrane juxtaposition and it thus provides a mechanism for promoting fusion (Figure 1). Peptides derived from this C-peptide helix (some as short as 12-16 residues) have been shown to bind the coiled coil and inhibit membrane fusion by HIV in human cells.[8-9] The best known of these Enfuvirtide (Fuzeon?) consists of 36-residues and is an FDA-approved treatment. However like other short peptide drugs its chemical synthesis is extraordinarily expensive and it exhibits poor serum stability (t1/2 ~3.8 hours). We reasoned that a helix-grafted alternative might retain similar specificity but have improved stability solubility and availability. Figure 1 (values of ~77 °C ~79 °C ~83 °C for 5-helix GLUE-Cpep and the complex respectively). The melting transition for the 1:1 sample is also highly cooperative further supporting a well-defined assembly. Figure 4 (were co- transformed with plasmids encoding 5-helix fused to the N- terminal half of spGFP (N-spGFP-5-helix) and one of two C-spGFP fusions: GLUE-Cpep or the gp41 C-peptide by itself. Interaction-dependent reassembly of GFP fragments (to Coumarin 7 generate a fluorescent signal) was measured by flow cytometry. Cells expressing either ligand construct are highly fluorescent in contrast to a control with nothing fused to C-spGFP (Figure 4D). We further characterized this interaction using an Enzyme-Linked Immunosorbant Assay (ELISA). The grafted GLUE binds 5-helix with slightly better affinity than the native C-peptide (Figure 4E columns 3 and 4 respectively) while the wild type GLUE exhibits no appreciable affinity (Figure 4E column 2) confirming the need for the grafted domain. This ELISA signal is observed even for a GLUE-Cpep sample that was pre-incubated with human serum (Supporting Information) confirming that the degradation-resistant form of the protein remains functional. Taken together these experiments show that the helix-grafted GLUE binds 5-helix in the context of a complex cellular milieu in a manner comparable to the native ligand and with improved serum longevity. Binding selectivity was assessed by measuring the amount of protein that is co-purified from expressing an untagged GLUE-Cpep (~17.1 kDa) and His6x-tagged 5-helix Mouse monoclonal to ENO2 (~25.4 kDa). Coumarin 7 As seen in Figure 4F the tagged 5-helix co-purifies with a single protein which was identified as GLUE-Cpep by mass spectrometry (Supporting Information Figure S2). The similar amounts of each co-purified protein (as determined by densitometry measurements of each protein band) further indicates that the complex involves a 1:1 ratio of proteins. The relatively miniscule levels of other co-purified cellular proteins indicates excellent selectivity for this interaction even in a complex cellular environment suggesting a reasonably strong mutual affinity. In conclusion we have demonstrated that the solvent exposed C-terminal alpha helix of the GLUE protein scaffold can be dramatically modified and extended so as to mimic the function of the gp41 C-peptide. ELISA and Coumarin 7 co-purification data indicate that GLUE-Cpep selectively binds 5-helix a protein that mimics the native C-peptide receptor. Unlike the isolated C-helix of Enfuvirtide GLUE-Cpep is soluble and well-folded in aqueous solution at room temperature (~25 °C) and is resistant to degradation in human serum at physiological temperature (~37 °C). Thus this protein drug lead overcomes challenges faced by traditional peptide reagents and may represent a new reagent for inhibition of HIV entry. Additionally helix-grafting onto PH and PH-like domains such as GLUE may be a general approach to the development of new reagents of interest to a diverse set of diseases that rely on helix-driven assembly. Finally GLUE-Cpep serves as a starting point for the generation of higher affinity and more selective mutants through the application of high-throughput screening or selection methods. Such experiments are currently underway and.