Memorizing spatiotemporal patterns

Single-molecule imaging revealed dynamic GPCR dimerization

Single fluorescent-molecule video imaging and tracking in living cells are revolutionizing our understanding of molecular interactions in the plasma membrane and intracellular membrane systems. They have revealed that molecular interactions occur surprisingly dynamically on much shorter time scales (≪1s) than those expected from the results by conventional techniques, such as pull-down assays (minutes to hours). Single-molecule imaging has unequivocally showed that G-protein-coupled receptors (GPCRs) undergo dynamic equilibrium between monomers and dimers, by enabling the determination of the 2D monomer-dimer equilibrium constant, the dimer dissociation rate constant (typically ∼10s(-1)), and the formation rate constant. Within one second, GPCRs typically undergo several cycles of monomer and homo-dimer formation with different partners.

Full characterization of GPCR monomer-dimer dynamic equilibrium by single molecule imaging

Receptor dimerization is important for many signaling pathways. However, the monomer-dimer equilibrium has never been fully characterized for any receptor with a 2D equilibrium constant as well as association/dissociation rate constants (termed super-quantification). Here, we determined the dynamic equilibrium for the N-formyl peptide receptor (FPR), a chemoattractant G protein-coupled receptor (GPCR), in live cells at 37°C by developing a single fluorescent-molecule imaging method. Both before and after liganding, the dimer-monomer 2D equilibrium is unchanged, giving an equilibrium constant of 3.6 copies/µm(2), with a dissociation and 2D association rate constant of 11.0 s(-1) and 3.1 copies/µm(2)s(-1), respectively. At physiological expression levels of ∼2.1 receptor copies/µm(2) (∼6,000 copies/cell), monomers continually convert into dimers every 150 ms, dimers dissociate into monomers in 91 ms, and at any moment, 2,500 and 3,500 receptor molecules participate in transient dimers and monomers, respectively. Not only do FPR dimers fall apart rapidly, but FPR monomers also convert into dimers very quickly.

JBIR-22, an inhibitor for protein-protein interaction of the homodimer of proteasome assembly factor 3

Proteasome assembling chaperone (PAC) 3 acts as a homodimer and plays an important role in proteasome formation. We screened JBIR-22 (1) as an inhibitor for protein-protein interaction (PPI) of PAC3 homodimer from our natural product library using a protein fragment complementation assay (PCA) with monomeric Kusabira-Green fluorescent protein (mKG) in vitro and found that 1 exhibited potent inhibitory activity against PAC3 homodimerization. Compound 1 showed long-term cytotoxicity against the human cervical carcinoma cell line, HeLa. This is the first report of a PPI inhibitor for proteasome assembly factors.

Novel in vitro protein fragment complementation assay applicable to high-throughput screening in a 1536-well format

Protein-protein interactions (PPIs) play key roles in all cellular processes and hence are useful as potential targets for new drug development. To facilitate the screening of PPI inhibitors as anticancer drugs, the authors have developed a high-throughput screening (HTS) system using an in vitro protein fragment complementation assay (PCA) with monomeric Kusabira-Green fluorescent protein (mKG). The in vitro PCA system was established by the topological formation of a functional complex between 2 split inactive mKG fragments fused to target proteins, which fluoresces when 2 target proteins interact to allow complementation of the mKG fragments. Using this assay system, the authors screened inhibitors for TCF7/beta-catenin, PAC1/PAC2, and PAC3 homodimer PPIs from 123,599 samples in their natural product library. Compound TB1 was identified as a specific inhibitor for PPI of PAC3 homodimer. TB1 strongly inhibited the PPI of PAC3 homodimer with an IC(50) value of 0.020 microM and did not inhibit PPI between TCF7/beta-catenin and PAC1/PAC2 even at a concentration of 250 microM. The authors thus demonstrated that this in vitro PCA system applicable to HTS in a 1536-well format is capable of screening for PPI inhibitors from a huge natural product library.

Monomeric fluorescent timers that change color from blue to red report on cellular trafficking

Based on the mechanism for chromophore formation in red fluorescent proteins, we developed three mCherry-derived monomeric variants, called fluorescent timers (FTs), that change their fluorescence from the blue to red over time. These variants exhibit distinctive fast, medium and slow blue-to-red chromophore maturation rates that depend on the temperature. At 37 degrees C, the maxima of the blue fluorescence are observed at 0.25, 1.2 and 9.8 h for the purified fast-FT, medium-FT and slow-FT, respectively. The half-maxima of the red fluorescence are reached at 7.1, 3.9 and 28 h, respectively. The FTs show similar timing behavior in bacteria, insect and mammalian cells. Medium-FT allowed for tracking of the intracellular dynamics of the lysosome-associated membrane protein type 2A (LAMP-2A) and determination of its age in the targeted compartments. The results indicate that LAMP-2A transport through the plasma membrane and early or recycling endosomes to lysosomes is a major pathway for LAMP-2A trafficking.