Increasingly, it is possible to design peptide and protein assemblies de novo from first principles or computationally. This provides new routes to functional synthetic polypeptides, including designs to target and bind proteins of interest. Much of this work has been developed in vitro. Therefore, a challenge is to deliver de novo polypeptides efficiently to sites of action within cells. Here, we describe the design, characterization, intracellular delivery, and subcellular localisation of a de novo synthetic peptide system. This comprises a dual-function basic peptide, programmed both for cell penetration and target binding, and a complementary acidic peptide that can be fused to proteins of interest and introduced into cells using synthetic DNA. The designs are characterized in vitro using biophysical methods and X-ray crystallography. The utility of the system for delivery into mammalian cells and subcellular targeting is demonstrated by marking organelles and actively engaging functional protein complexes.

Supramolecular signaling assemblies are of interest for their unique signaling properties. A µm scale signaling assembly, the central supramolecular signaling cluster (cSMAC), forms at the center of the interface of T cells activated by antigen-presenting cells. We have determined that it is composed of multiple complexes of a supramolecular volume of up to 0.5 µm3 and associated with extensive membrane undulations. To determine cSMAC function, we have systematically manipulated the localization of three adaptor proteins, LAT, SLP-76, and Grb2. cSMAC localization varied between the adaptors and was diminished upon blockade of the costimulatory receptor CD28 and deficiency of the signal amplifying kinase Itk. Reconstitution of cSMAC localization restored IL-2 secretion which is a key T cell effector function as dependent on reconstitution dynamics. Our data suggest that the cSMAC enhances early signaling by facilitating signaling interactions and attenuates signaling thereafter through sequestration of a more limited set of signaling intermediates.

De novo peptides and proteins that switch state in response to chemical and physical cues would advance protein design and synthetic biology. Here we report two designed systems that disassemble and reassemble upon site-specific phosphorylation and dephosphorylation, respectively. As starting points, we use hyperthermostable de novo antiparallel and parallel coiled-coil heterotetramers, i.e., A 2 B 2 systems, to afford control in downstream applications. The switches are incorporated by adding protein kinase A phosphorylation sites, R-R-X-S, with the phosphoacceptor serine residues placed to maximize disruption of the coiled-coil interfaces. The unphosphorylated peptides assemble as designed and unfold reversibly when heated. Addition of kinase to the assembled states unfolds them with half-lives of ≤5 min. Phosphorylation is reversed by Lambda Protein Phosphatase resulting in tetramer reassembly. We envisage that the new de novo designed coiled-coil components, the switches, and a mechanistic model for them will be useful in synthetic biology, biomaterials, and biotechnology applications.

Supramolecular signaling assemblies are of interest for their unique signaling properties. A µm scale signaling assembly, the central supramolecular signaling cluster (cSMAC), forms at the center interface of T cells activated by antigen presenting cells (APC). The adaptor protein linker for activation of T cells (LAT) is a key cSMAC component. The cSMAC has widely been studied using total internal reflection fluorescence microscopy of CD4 + T cells activated by planar APC substitutes. Here we provide a protocol to image the cSMAC in its cellular context at the interface between a T cell and an APC. Super resolution stimulated emission depletion microscopy (STED) was utilized to determine the localization of LAT, that of its active, phosphorylated form and its entire pool. Agonist peptide-loaded APCs were incubated with TCR transgenic CD4 + T cells for 4.5 min before fixation and antibody staining. Fixed cell couples were imaged using a 100x 1.4 NA objective on a Leica SP8 AOBS confocal laser scanning microscope. LAT clustered in multiple supramolecular complexes and their number and size distributions were determined. Using this protocol, cSMAC properties in its cellular context at the interface between a T cell and an APC could be quantified.

No abstract