Site-specific C-terminal and internal loop labeling of proteins using sortase-mediated reactions

Methods for site-specific modification of proteins should be quantitative and versatile with respect to the nature and size of the biological or chemical targets involved. They should require minimal modification of the target, and the underlying reactions should be completed in a reasonable amount of time under physiological conditions. Sortase-mediated transpeptidation reactions meet these criteria and are compatible with other labeling methods. Here we describe the expression and purification conditions for two sortase A enzymes that have different recognition sequences. We also provide a protocol that allows the functionalization of any given protein at its C terminus, or, for select proteins, at an internal site. The target protein is engineered with a sortase-recognition motif (LPXTG) at the place where modification is desired. Upon recognition, sortase cleaves the protein between the threonine and glycine residues, facilitating the attachment of an exogenously added oligoglycine peptide modified with the functional group of choice (e.g., fluorophore, biotin, protein or lipid). Expression and purification of sortase takes ∼3 d, and sortase-mediated reactions take only a few minutes, but reaction times can be extended to increase yields.National Institutes of Health (U.S.) (Grant RO1 AI08787

Site-Specific Chemoenzymatic Labeling of Aerolysin Enables the Identification of New Aerolysin Receptors

Aerolysin is a secreted bacterial toxin that perforates the plasma membrane of a target cell with lethal consequences. Previously explored native and epitope-tagged forms of the toxin do not allow site-specific modification of the mature toxin with a probe of choice. We explore sortase-mediated transpeptidation reactions (sortagging) to install fluorophores and biotin at three distinct sites in aerolysin, without impairing binding of the toxin to the cell membrane and with minimal impact on toxicity. Using a version of aerolysin labeled with different fluorophores at two distinct sites we followed the fate of the C-terminal peptide independently from the N-terminal part of the toxin, and show its loss in the course of intoxication. Making use of the biotinylated version of aerolysin, we identify mesothelin, urokinase plasminogen activator surface receptor (uPAR, CD87), glypican-1, and CD59 glycoprotein as aerolysin receptors, all predicted or known to be modified with a glycosylphosphatidylinositol anchor. The sortase-mediated reactions reported here can be readily extended to other pore forming proteins.National Institutes of Health (U.S.) (grant R01 AI087879

Site-specific protein modification using immobilized sortase in batch and continuous-flow systems

Transpeptidation catalyzed by ​sortase A allows the preparation of proteins that are site-specifically and homogeneously modified with a wide variety of functional groups, such as fluorophores, PEG moieties, lipids, glycans, bio-orthogonal reactive groups and affinity handles. This protocol describes immobilization of ​sortase A on a solid support (Sepharose beads). Immobilization of ​sortase A simplifies downstream purification of a protein of interest after labeling of its N or C terminus. Smaller batch and larger-scale continuous-flow reactions require only a limited amount of enzyme. The immobilized enzyme can be reused for multiple cycles of protein modification reactions. The described protocol also works with a Ca²⁺-independent variant of ​sortase A with increased catalytic activity. This heptamutant variant of ​sortase A (7M) was generated by combining previously published mutations, and this immobilized enzyme can be used for the modification of calcium-senstive substrates or in instances in which low temperatures are needed. Preparation of immobilized ​sortase A takes 1–2 d. Batch reactions take 3–12 h and flow reactions proceed at 0.5 ml h⁻¹, depending on the geometry of the reactor used.United States. National Institutes of Health (RO1 AI087879

Monovalent engagement of the BCR activates ovalbumin-specific transnuclear B cells

Valency requirements for B cell activation upon antigen encounter are poorly understood. OB1 transnuclear B cells express an IgG1 B cell receptor (BCR) specific for ovalbumin (OVA), the epitope of which can be mimicked using short synthetic peptides to allow antigen-specific engagement of the BCR. By altering length and valency of epitope-bearing synthetic peptides, we examined the properties of ligands required for optimal OB1 B cell activation. Monovalent engagement of the BCR with an epitope-bearing 17-mer synthetic peptide readily activated OB1 B cells. Dimers of the minimal peptide epitope oriented in an N to N configuration were more stimulatory than their C to C counterparts. Although shorter length correlated with less activation, a monomeric 8-mer peptide epitope behaved as a weak agonist that blocked responses to cell-bound peptide antigen, a blockade which could not be reversed by CD40 ligation. The 8-mer not only delivered a suboptimal signal, which blocked subsequent responses to OVA, anti-IgG, and anti-kappa, but also competed for binding with OVA. Our results show that fine-tuning of BCR-ligand recognition can lead to B cell nonresponsiveness, activation, or inhibition

Production of unnaturally linked chimeric proteins using a combination of sortase-catalyzed transpeptidation and click chemistry

Chimeric proteins, including bispecific antibodies, are biological tools with therapeutic applications. Genetic fusion and ligation methods allow the creation of N-to-C and C-to-N fused recombinant proteins, but not unnaturally linked N-to-N and C-to-C fusion proteins. This protocol describes a simple procedure for the production of such chimeric proteins, starting from correctly folded proteins and readily available peptides. By equipping the N terminus or C terminus of the proteins of interest with a set of click handles using sortase A, followed by a strain-promoted click reaction, unnatural N-to-N and C-to-C linked (hetero) fusion proteins are established. Examples of proteins that have been conjugated via this method include interleukin-2, interferon-alpha, ubiquitin, antibodies and several single-domain antibodies. If the peptides, sortase A and the proteins of interest are in hand, the unnaturally N-to-N and C-to-C fused proteins can be obtained in 3-4 d

Rapid Capture and Labeling of Cells on Single Domain Antibodies-Functionalized Flow Cell

Current techniques to characterize leukocyte subgroups in blood require long sample preparation times and sizable sample volumes. A simplified method for leukocyte characterization using smaller blood volumes would thus be useful in diagnostic settings. Here we describe a flow system comprised of two functionalized graphene oxide (GO) surfaces that allow the capture of distinct leukocyte populations from small volumes blood using camelid single-domain antibodyfragments (VHHs) as capture agents. We used site-specifically labeled leukocytes to detect and identify cells exposed to fungal challenge. Combining the chemical and optical properties of GO with the versatility of the VHH scaffold in the context of a flow system provides a quick and efficient method for the capture and characterization of functional leukocytes. Keywords: Single domain antibody, Graphene oxide, Sortase, Enzymatic labeling, Cell detection, LeukocytesNational Institutes of Health (U.S.) (NIH Grant no. 4DP1GM106409)National Institutes of Health (U.S.) (NIH Grant no. R01AI87879)Taiwan. Ministry of Science and Technology (MOST 105-2633-B-009-003)Taiwan. Ministry of Science and Technology (MOST 105-2628-B-009-001-MY3)National Chiao Tung University (Taiwan) (105W970

Engineered red blood cells as carriers for systemic delivery of a wide array of functional probes

We developed modified RBCs to serve as carriers for systemic delivery of a wide array of payloads. These RBCs contain modified proteins on their plasma membrane, which can be labeled in a sortase-catalyzed reaction under native conditions without inflicting damage to the target membrane or cell. Sortase accommodates a wide range of natural and synthetic payloads that allow modification of RBCs with substituents that cannot be encoded genetically. As proof of principle, we demonstrate site-specific conjugation of biotin to in vitro-differentiated mouse erythroblasts as well as to mature mouse RBCs. Thus modified, RBCs remain in the bloodstream for up to 28 d. A single domain antibody attached enzymatically to RBCs enables them to bind specifically to target cells that express the antibody target. We extend these experiments to human RBCs and demonstrate efficient sortase-mediated labeling of in vitro-differentiated human reticulocytes.United States. Defense Advanced Research Projects Agency (Contract HR0011-12-2-0015

Usp12 stabilizes the T-cell receptor complex at the cell surface during signaling

Posttranslational modifications are central to the spatial and temporal regulation of protein function. Among others, phosphorylation and ubiquitylation are known to regulate proximal T-cell receptor (TCR) signaling. Here we used a systematic and unbiased approach to uncover deubiquitylating enzymes (DUBs) that participate during TCR signaling in primary mouse T lymphocytes. Using a C-terminally modified vinyl methyl ester variant of ubiquitin (HA-Ub-VME), we captured DUBs that are differentially recruited to the cytosol on TCR activation. We identified ubiquitin-specific peptidase (Usp) 12 and Usp46, which had not been previously described in this pathway. Stimulation with anti-CD3 resulted in phosphorylation and time-dependent translocation of Usp12 from the nucleus to the cytosol. Usp12−/− Jurkat cells displayed defective NFκB, NFAT, and MAPK activities owing to attenuated surface expression of TCR, which were rescued on reconstitution of wild type Usp12. Proximity-based labeling with BirA-Usp12 revealed several TCR adaptor proteins acting as interactors in stimulated cells, of which LAT and Trat1 displayed reduced expression in Usp12−/− cells. We demonstrate that Usp12 deubiquitylates and prevents lysosomal degradation of LAT and Trat1 to maintain the proximal TCR complex for the duration of signaling. Our approach benefits from the use of activity-based probes in primary cells without any previous genome modification, and underscores the importance of ubiquitin-mediated regulation to refine signaling cascades.Institut Pasteur International NetworkUniversity Grants Committee (Hong Kong, China) (Grant AoE/M-12/05

Fluorophore-Conjugated Holliday Junctions for Generating Super-Bright Antibodies and Antibody Fragments

The site-specific modification of proteins with fluorophores can render a protein fluorescent without compromising its function. To avoid self-quenching from multiple fluorophores installed in close proximity, we used Holliday junctions to label proteins site-specifically. Holliday junctions enable modification with multiple fluorophores at reasonably precise spacing. We designed a Holliday junction with three of its four arms modified with a fluorophore of choice and the remaining arm equipped with a dibenzocyclooctyne substituent to render it reactive with an azide-modified fluorescent single-domain antibody fragment or an intact immunoglobulin produced in a sortase-catalyzed reaction. These fluorescent Holliday junctions improve fluorescence yields for both single-domain and full-sized antibodies without deleterious effects on antigen binding