Nef-mediated internalization of T-cell receptor molecules from the surface of an infected cell is required for the pathogenicity of HIV and disease progression to AIDS. This function depends on the N-terminal myristoylation of Nef, a lipid modification that targets the protein to membranes. We have analyzed how specific membrane properties and sequence motifs within Nef determine this interaction. Using time-resolved techniques we find that the association with membranes is a biphasic process with a fast rate for an electrostatic-driven protein-liposome interaction and a slow rate for the formation of an amphipathic helix. The rate of myristate insertion into liposomes depends on membrane curvature, while changes in the lipid composition with respect to phosphoinositides, cholesterol or sphingomyelin did not significantly alter the interaction. Moreover, Nef binding to membranes requires negatively charged liposomes, and mutations of basic and hydrophobic residues strongly diminished the association and changed the binding kinetics differently.

Previously, we reported the manual assembly of lipid-coated aqueous droplets in oil to form two-dimensional (2D) networks in which the droplets are connected through single lipid bilayers. Here we assemble lipid-coated droplets in robust, freestanding 3D geometries: for example, a 14-droplet pyramidal assembly. The networks are designed, and each droplet is placed in a designated position. When protein pores are inserted in the bilayers between specific constituent droplets, electrical and chemical communication pathways are generated. We further describe an improved means to construct 3D droplet networks with defined organizations by the manipulation of aqueous droplets containing encapsulated magnetic beads. The droplets are maneuvered in a magnetic field to form simple construction modules, which are then used to form larger 2D and 3D structures including a 10-droplet pyramid. A methodology to construct freestanding, functional 3D droplet networks is an important step toward the programmed and automated manufacture of synthetic minimal tissues.

The accessory HIV-1 Nef protein is essential for viral replication, high virus load, and progression to AIDS. These functions are mediated by the alteration of signaling and trafficking pathways and require the membrane association of Nef by its N-terminal myristoylation. However, a large portion of Nef is also found in the cytosol, in line with the observation that myristoylation is only a weak lipidation anchor for membrane attachment. We performed biochemical studies to analyze the implications of myristoylation on the conformation of Nef in aqueous solution. To establish an in vivo myristoylation assay, we first optimized the codon usage of Nef for Escherichia coli expression, which resulted in a 15-fold higher protein yield. Myristoylation was achieved by coexpression with the N-myristoyltransferase and confirmed by mass spectrometry. The myristoylated protein was soluble, and proton NMR spectra confirmed proper folding. Size exclusion chromatography revealed that myristoylated Nef appeared of smaller size than the unmodified form but not as small as an N-terminally truncated from of Nef that omits the anchor domain. Western blot stainings and limited proteolysis of both forms showed different recognition profiles and degradation pattern. Analytical ultracentrifugation revealed that myristoylated Nef prevails in a monomeric state while the unmodified form exists in an oligomeric equilibrium of monomer, dimer, and trimer associations. Finally, fluorescence correlation spectroscopy using multiphoton excitation revealed a shorter diffusion time for the lipidated protein compared to the unmodified form. Taken together, our data indicated myristoylation-dependent conformational changes in Nef, suggesting a rather compact and monomeric form for the lipidated protein in solution.