Mechanisms of protein recognition have been extensively studied for single-domain proteins1, but are less well characterized for dynamic multi-domain systems. Ubiquitin (Ub) chains represent a biologically important multi-domain system that requires recognition by structurally diverse Ub-interacting proteins (UbIPs)2,3. Ub chain conformations in isolation are often different from conformations observed in UbIP complexes, suggesting either great dynamic flexibility or extensive chain remodeling upon binding.Using single-molecule FRET, we show here that Lys63-, Lys48- and Met1-linked diUb exist in several distinct conformational states in solution. Lys63- and Met1-linked diUb adopt extended ‘open’ and more compact ‘closed’ conformations, and Ub binding domains (UBDs) and deubiquitinases (DUBs) select pre-existing conformations. In contrast, Lys48-linked diUb adopts predominantly compact conformations. DUBs directly recognize existing conformations, but may also remodel Ub chains to hydrolyze the isopeptide bond. Disruption of the Lys48-diUb interface changes conformational dynamics and affects DUB activity. Hence, conformational equilibria in Ub chains provide an additional layer of regulation in the Ub system, and distinct conformations observed in differently linked polyUb may contribute to the specificity of UbIPs.

We report a microfluidic droplet-based approach enabling the measurement of chemical reactions of individual enzyme molecules and its application to a single-molecule-counting immunoassay. A microfluidic device is used to generate and manipulate <10 fL droplets at rates of up to 1.3 × 10(6) per second, about 2 orders of magnitude faster than has previously been reported. The femtodroplets produced with this device can be used to encapsulate single biomolecular complexes tagged with a reporter enzyme; their small volume enables the fluorescent product of a single enzyme molecule to be detected within 10 min of on-chip incubation. Our prototype system is validated by detection of a biomarker for prostate cancer in buffer, down to a concentration of 46 fM. This work demonstrates a highly flexible and sensitive diagnostic platform that exploits extremely high-speed generation of monodisperse femtoliter droplets for the counting of individual analyte molecules.

Immunogold staining and electron microscopy show that IL-2 receptor ␣-subunits exhibit nonrandom surface distribution on human T lymphoma cells. Analysis of interparticle distances reveals that this clustering on the scale of a few hundred nanometers is independent of the presence of IL-2 and of the expression of the IL-2R ␤-subunit. Clustering of IL-2R␣ is confirmed by confocal microscopy, yielding the same average cluster size, Ϸ600 -800 nm, as electron microscopy. HLA class I and II and CD48 molecules also form clusters of the same size. Disruption of cholesterol-rich lipid rafts with filipin or depletion of membrane cholesterol with methyl-␤-cyclodextrin results in the blurring of cluster boundaries and an apparent dispersion of clusters for all four proteins. Interestingly, the transferrin receptor, which is thought to be located outside lipid rafts, exhibits clusters that are only 300 nm in size and are less affected by modifying the membrane cholesterol content. Furthermore, transferrin receptor clusters hardly colocalize with IL-2R␣, HLA, and CD48 molecules (crosscorrelation coefficient is 0.05), whereas IL-2R␣ colocalizes with both HLA and CD48 (crosscorrelation coefficient is between 0.37 and 0.46). This coclustering is confirmed by electron microscopy. The submicron clusters of IL-2R␣ chains and their coclustering with HLA and CD48, presumably associated with lipid rafts, could underlie the efficiency of signaling in lymphoid cells.IL-2 receptor ͉ HLA glycoproteins ͉ transferrin receptor ͉ receptor clustering ͉ electron microscopy