Multiplexed single-molecule characterization at the library scale

Single-molecule techniques are exceptionally well suited for analyzing the complex dynamic behavior of macromolecules involved in fundamental biological processes. Nevertheless, time and cost usually restrict current single-molecule methods to examining a limited number of different samples. At the same time, a broad sequence or chemical space often needs to be investigated to gain a thorough understanding of complex biological phenomena. To address this urgent need, we have developed multiplexed single-molecule characterization at the library scale (MUSCLE), a method that combines single-molecule fluorescence microscopy with next-generation sequencing to enable highly multiplexed observations of complex dynamics on millions of individual molecules spanning thousands of distinct sequences or barcoded entities. In this protocol, we outline the implementation of MUSCLE and present examples from our recent research, such as the sequence-dependent dynamics of Cas9-induced target DNA unwinding and rewinding. This example demonstrates that MUSCLE can be applied to study protein-nucleic acid interactions, going beyond nucleic-acid-only model systems. We detail the sample and library design, high-throughput single-molecule data acquisition, next-generation sequencing, spatial registration of single-molecule fluorescence and sequencing data and downstream data analysis. The ligation-based surface immobilization approach of MUSCLE ensures high clustering efficiency (>40%), increasing throughput and simplifying registration. In addition, MUSCLE includes a 3D-printed flow cell adapter that enables liquid exchange during single-molecule fluorescence microscopy. The complete procedure typically spans 3-4 days and yields a dataset that comprehensively characterizes the dynamic behavior of a library of constructs.