Site-Selective Protein Conjugation by a Multicomponent Ugi Reaction

The chemical bioconjugation of proteins has seen tremendous applications in the past decades, with the booming of antibody-drug conjugates and their use in oncology. While genetic engineering has permitted to produce bespoke proteins featuring key (un-)natural amino acid residues poised for site-selective modifications, the conjugation of native proteins is riddled with selectivity issues. Chemoselective strategies are plentiful and enable the precise modification of virtually any residue with a reactive side-chain; site-selective methods are less common and usually most effective on small and medium-sized proteins. In this context, we studied the application of the Ugi multicomponent reaction for the site-selective conjugation of amine and carboxylate groups on proteins, and antibodies in particular. Through an in-depth mechanistic methodology work supported by peptide mapping studies, we managed to develop a set of conditions allowing the highly selective modification of antibodies bearing N-terminal glutamate and aspartate residues. We demonstrated that this strategy did not alter their affinity toward their target antigen and produced an antibody-drug conjugate with subnanomolar potency. Excitingly, we showed that the high site selectivity of our strategy was maintained on other protein formats, especially on anticalins, for which directed mutagenesis helped to highlight the key importance of a single lysine residue.

Cross-validation of ELISA and a portable surface plasmon resonance instrument for IgG antibody serology with SARS-CoV-2 positive individuals

We report on the development of surface plasmon resonance (SPR) sensors and matching ELISAs for the detection of nucleocapsid and spike antibodies specific against the novel coronavirus 2019 (SARS-CoV-2) in human serum, plasma and dried blood spots (DBS). When exposed to SARS-CoV-2 or a vaccine against SARS-CoV-2, the immune system responds by expressing antibodies at levels that can be detected and monitored to identify the fraction of the population potentially immunized against SARS-CoV-2 and support efforts to deploy a vaccine strategically. A SPR sensor coated with a peptide monolayer and functionalized with various sources of SARS-CoV-2 recombinant proteins expressed in different cell lines detected human anti-SARS-CoV-2 IgG antibodies in clinical samples. Nucleocapsid expressed in different cell lines did not significantly change the sensitivity of the assays, whereas the use of a CHO cell line to express spike ectodomain led to excellent performance. This bioassay was performed on a portable SPR instrument capable of measuring 4 biological samples within 30 minutes of sample/sensor contact and the chip could be regenerated at least 9 times. Multi-site validation was then performed with in-house and commercial ELISA, which revealed excellent cross-correlations with Pearson's coefficients exceeding 0.85 in all cases, for measurements in DBS and plasma. This strategy paves the way to point-of-care and rapid testing for antibodies in the context of viral infection and vaccine efficacy monitoring.

The Structure−Activity Relationship of Ferric Pyoverdine Bound to Its Outer Membrane Transporter: Implications for the Mechanism of Iron Uptake

Under iron limitation, Pseudomonas aeruginosa ATCC 15692 secretes a major siderophore, pyoverdine I (PvdI). This molecule chelates iron in the extracellular medium and shuttles it into the cells via a specific outer membrane transporter, FpvAI. PvdI consists of a fluorescent chromophore derived from 2,3-diamino-6,7-dihydroxyquinoline and containing one of the bidentate groups involved in iron chelation, linked to a peptide moiety containing the two other bidentate groups required for binding to Fe(3+). Kinetic studies, based on the fluorescence properties of this siderophore, showed that pH 8.0 was optimal for the binding of PvdI and PvdI-Fe to FpvAI. We investigated the mechanism of interaction of PvdI and PvdI-Fe with FpvAI, by synthesizing various analogues of this siderophore, determining their affinity for FpvAI in vitro and in vivo and their ability to transport iron, and interpreting the results obtained in light of the structure of FpvAI-PvdI. Our findings demonstrate that the succinyl moiety linked to the chromophore of PvdI and the first amino acid of the peptide moiety can be sterically hindered with no effect on binding or the iron uptake properties of PvdI-Fe. Moreover, the sequence and the structure of the peptide moiety of PvdI seems to be more important for the iron uptake step than for the binding of the siderophore to FpvAI. Finally, the efficiency of iron uptake and of recycling of the various PvdI analogues after iron release suggests that iron dissociates from PvdI on FpvAI or in the periplasm. All these data have serious implications for the specificity and mechanism of PvdI-mediated iron transport in P. aeruginosa.