The rapidly increasing application of antibodies has inspired the development of several novel methods to isolate and target antibodies using smart biomaterials that mimic the binding of Fc-receptors to antibodies. The Fc-binding domain of antibodies is the primary binding site for e.g., effector proteins and secondary antibodies, whereas antigens bind to the Fab region. Protein A, G, and L, surface proteins expressed by pathogenic bacteria, are well known to bind immunoglobulin and have been widely exploited in antibody purification strategies. Several difficulties are encountered when bacterial proteins are used in antibody research and application. One of the major obstacles hampering the use of bacterial proteins is sample contamination with trace amounts of these proteins, which can invoke an immune response in the host. Many research groups actively develop synthetic ligands that are able to selectively and strongly bind to antibodies. Among the reported ligands, peptides that bind to the Fc-domain of antibodies are attractive tools in antibody research. Besides their use as high affinity ligands in antibody purification chromatography, Fc-binding peptides are applied e.g., to localize antibodies on nanomaterials and to increase the half-life of proteins in serum. In this review, recent developments of Fc-binding peptides are presented and their binding characteristics and diverse applications are discussed.

A highly selective detection method of native protein tyrosine phosphatase 1B (PTP1B) is described using a target specific probe equipped with 1‐naphthylamine (λex=330 nm, λem=445 nm). Irradiation of a mixture of PTP1B and Probe 1 with ultraviolet light of 280 nm (corresponding to PTP1B excitation maximum) resulted in significant fluorescence increase at 445 nm, following FRET characteristics. This phenomenon does not occur with other closely related phosphatases or cellular abundant alkaline phosphatase (APP). Probe 1, the most potent and selective probe, was found to competitively inhibit PTP1B (Ki≈42 nm), whereas APP inhibition was found to be in the low micromolar range. Furthermore, Probe 1 discriminates between PTP1B and several other phosphatases. Here, we report real‐time label‐free FRET detection of pure PTP1B as well as induced human PTP1B in Escherichia coli cell lysate. In contrast to 6,8‐difluoro‐4‐methylumbelliferyl phosphate (DiFMUP), a representative fluorescence turn‐on PTP substrate, our FRET probe successfully differentiated human cervical carcinoma cell lysate, SiHa, which has a high expression level of PTP1B, from PTP1B‐knockdown SiHa cell lysate (that is, siRNA was used for PTP1B knockdown).

The protein p73 acts as a transcription factor, resulting in tumour suppression. MDM2, an oncogenic protein, can negatively influence p73‐mediated apoptosis by binding to p73 transactivation domains (TAD). Inhibition of the protein‐protein interaction between p73 and oncogenic proteins is an attractive strategy for promoting p73‐mediated apoptosis. Herein, we describe the use of a modified p73‐TAD peptide for the FRET‐based assay of the binding of p73‐TAD to MDM2. The FRET probe, equipped with 1‐naphthylamine (λex=330 nm, λem=445 nm), serves as a FRET acceptor, and the tryptophan of the protein acts as FRET donor (λex=280 nm, λem=340 nm). Sensitized emission from the FRET probe was observed upon excitation of the protein‐FRET‐probe complex at the excitation wavelength of Trp. Furthermore, addition of the MDM2 inhibitor Nutiln‐3 drastically reduced the FRET signal, thus indicating that the FRET probe competes with Nutiln‐3 for MDM2 binding. The developed FRET binding assay might be applicable in high‐throughput screening of novel drugs that inhibit interactions between p73 and MDM2.

Silica nanoparticles (SiNPs, 30 and 70 nm) and microparticles (SiMPs, 10 μm) were functionalized with a PTP1B specific probe for label‐free fluorescence detection of PTP1B. Intrinsic tryptophan residues of PTP1B serve as the Förster resonance energy transfer (FRET) donor and the functionalized silica particles as the FRET acceptor. When a mixture of the functionalized SiNP and PTP1B is excited at tryptophan excitation maximum (280 nm), significant sensitized fluorescence was detected at 450 nm as a result of FRET. The limit of detection of PTP1B was found to be 29 and 70 nM for the functionalized 30 and 70 nm SiNP, respectively. Using this approach, PTP1B could be detected in E. coli cell lysate in which PTP1B was induced. The functionalized SiNP probe (SiNP/NA) was selective toward PTP1B over BSA, and the nanoparticles are able to penetrate into cells. Furthermore, the binding of PTP1B to the SiMPs could be visualized under a deep UV‐microscope.