Cellular processes can only be understood as the dynamic interplay of molecules. There is a need for techniques to monitor interactions of endogenous proteins directly in individual cells and tissues to reveal the cellular and molecular architecture and its responses to perturbations. Here we report our adaptation of the recently developed proximity ligation method to examine the subcellular localization of protein-protein interactions at single-molecule resolution. Proximity probes-oligonucleotides attached to antibodies against the two target proteins-guided the formation of circular DNA strands when bound in close proximity. The DNA circles in turn served as templates for localized rolling-circle amplification (RCA), allowing individual interacting pairs of protein molecules to be visualized and counted in human cell lines and clinical specimens. We used this method to show specific regulation of protein-protein interactions between endogenous Myc and Max oncogenic transcription factors in response to interferon-gamma (IFN-gamma) signaling and low-molecular-weight inhibitors.

The advent of in vitro DNA amplification has enabled rapid acquisition of genomic information. We present here an analogous technique for protein detection, in which the coordinated and proximal binding of a target protein by two DNA aptamers promotes ligation of oligonucleotides linked to each aptamer affinity probe. The ligation of two such proximity probes gives rise to an amplifiable DNA sequence that reflects the identity and amount of the target protein. This proximity ligation assay detects zeptomole (40 x 10(-21) mol) amounts of the cytokine platelet-derived growth factor (PDGF) without washes or separations, and the mechanism can be generalized to other forms of protein analysis.

Efficient and precise detection techniques, along with extensive repertoires of specific binding reagents, will be needed to meet the challenges of proteome analyses. The recently established proximity ligation mechanism enables sensitive high-capacity protein measurements by converting the detection of specific proteins to the analysis of DNA sequences. Proximity probes containing oligonucleotide extensions are designed to bind pairwise to target proteins and to form amplifiable tag sequences by ligation when brought in proximity. In our previous report, both the ligatable arms and the protein binders were DNA molecules. We now generalize the method by providing simple and convenient protocols to convert any polyclonal antibodies or matched pair of monoclonal antibodies to proximity probe sets through the attachment of oligonucleotide sequences. Sufficient reagent for >100,000 proximity ligation assays can be prepared from 1 g of antibody. The technique is applied to measure cytokines in a homogenous test format with femtomolar detection sensitivities in 1-l samples, and we exemplify its utility in situations when only minute sample amounts are available.

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We describe a scheme for biomolecule enumeration by converting nanometer-scale specific molecular recognition events mediated by rolling-circle amplification to fluorescent micrometer-sized DNA molecules amenable to discrete optical detection. Our amplified single-molecule detection (SMD) approach preserves the discrete nature of the molecular population, allowing multiplex detection and highly precise quantification of molecules over a dynamic range of seven orders of magnitude. We apply the method for sensitive detection and quantification of the bacterial pathogen Vibrio cholerae.