Effects of various spacers between biotin and the phospholipid headgroup on immobilization and sedimentation of biotinylated phospholipid-containing liposomes facilitated by avidin-biotin interactions

The Nedd4L ubiquitin ligase is activated by FCHO2-generated membrane curvature

The C2-WW-HECT domain ubiquitin ligase Nedd4L regulates membrane sorting during endocytosis through the ubiquitination of cargo molecules such as the epithelial sodium channel (ENaC). Nedd4L is catalytically autoinhibited by an intramolecular interaction between its C2 and HECT domains, but the protein's activation mechanism is poorly understood. Here, we show that Nedd4L activation is linked to membrane shape by FCHO2, a Bin-Amphiphysin-Rsv (BAR) domain protein that regulates endocytosis. FCHO2 was required for the Nedd4L-mediated ubiquitination and endocytosis of ENaC, with Nedd4L co-localizing with FCHO2 at clathrin-coated pits. In cells, Nedd4L was specifically recruited to, and activated by, the FCHO2 BAR domain. Furthermore, we reconstituted FCHO2-induced recruitment and activation of Nedd4L in vitro. Both the recruitment and activation were mediated by membrane curvature rather than protein-protein interactions. The Nedd4L C2 domain recognized a specific degree of membrane curvature that was generated by the FCHO2 BAR domain, with this curvature directly activating Nedd4L by relieving its autoinhibition. Thus, we show for the first time a specific function (i.e., recruitment and activation of an enzyme regulating cargo sorting) of membrane curvature by a BAR domain protein.

CD4+ T cell-targeting immunoliposomes for treatment of latent HIV reservoir

Active targeting nano-delivery is a promising approach to enhance therapeutic efficacy and specificity to the target cells. Liposomes (LPs) have been widely studied for the active targeting delivery due to their low toxicity, biodegradability, biocompatibility, and feasibility of surface medication to provide the interactions with cell receptors. One of the strategies is to functionalize the surface of LPs with monoclonal antibodies (mAbs) to obtain immunoliposomes (imLPs) that recognize specific receptors on target cells. Among several target cells, CD4+ T cells are known for playing a pivotal role in controlling the immune system in several diseases, including cancers, inflammatory diseases, and viral infections, particularly HIV-1. Here, we demonstrate two methods for conjugating αCD4 mAb with imLPs for specific targeting of CD4+ T cells that can harbor viral genome and serve as a predominant latent HIV reservoir. LPs conjugated with αCD4 mAb via neutravidin-biotin linkage were used for selectively targeting CD4+ J-Lat 10.6 cells. We demonstrate, via flow cytometry, the importance of the conjugation step, mAb density, and the presence of polyethylene glycol (PEG) for effective drug delivery to CD4+ T cells. The cellular uptake of imLPs is substantially higher if the imLPs are functionalized with the pre-conjugated αCD4 mAb-neutravidin complex. Furthermore, imLPs loaded with HIV-1 latency reversing agent, suberoylanilide hydroxamic acid (SAHA), could reactivate the J-Lat 10.6 cells, suggesting that the αCD4-imLPs could be potentially used as a targeted drug delivery system for HIV-1 latency reactivation or other CD4-targeted immunotherapies.

The role of the biotin linker in polymer antibody mimetics, iBodies, in biochemical assays

We investigated the effect of a linker between the polymer backbone and biotin, and the influence of the number of biotins per polymer chain as well as the biotin position on the polymer chain on the efficacy of the ELISA or pull-down assays.

Functional expression of monomeric streptavidin and fusion proteins in Escherichia coli: applications in flow cytometry and ELISA

Monomeric streptavidin (mSA) offers a combination of structural and binding properties that are useful in many applications, including a small size and monovalent biotin binding. Because mSA contains a structurally important disulfide bond, the molecule does not fold correctly when expressed inside the cell. We show that mSA can be expressed in a functional form in Escherichia coli by fusing the OmpA signal sequence at the amino terminus. Expressed mSA is exported to the periplasm, from which the molecule leaks to the medium under vigorous shaking. Purified mSA can be conjugated with FITC and used to label microbeads and yeast cells for analysis by flow cytometry, further expanding the scope of mSA-based applications. Some applications require recombinant fusion of mSA with another protein. mSA fused to EGFP cannot be secreted to the medium but was successfully expressed in an engineered cell line that supports oxidative folding in the cytoplasm. Purified mSA-EGFP and mSA-mCherry bound biotin with high affinity and were successfully used in conventional flow cytometry and imaging flow cytometry. Finally, we demonstrate the use of mSA in ELISA, in which horseradish peroxidase-conjugated mSA and biotinylated secondary antibody are used together to detect primary antibody captured on an ELISA plate. Engineering mSA to introduce additional lysine residues can increase the reporter signal above that of wild-type streptavidin. Together, these examples establish mSA as a convenient reagent with a potentially unique role in biotechnology.