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Methods Enzymol. 2019;616:43-59. doi: 10.1016/bs.mie.2018.10.031. Epub 2018 Dec 17.

Sortase-mediated fluorescent labeling of CRISPR complexes.

Author information

1
Department of Molecular Biosciences and Institute for Cellular and Molecular Biology, University of Texas at Austin, Austin, TX, United States.
2
Department of Molecular Biology and Genetics, Cornell University, Ithaca, NY, United States.
3
Department of Chemistry, University of Texas at Austin, Austin, TX, United States.
4
Department of Molecular Biosciences and Institute for Cellular and Molecular Biology, University of Texas at Austin, Austin, TX, United States; Center for Systems and Synthetic Biology, University of Texas at Austin, Austin, TX, United States. Electronic address: ifinkelstein@cm.utexas.edu.

Abstract

Fluorescent labeling of proteins is a critical requirement for single-molecule imaging studies. Many protein labeling strategies require harsh conditions or large epitopes that can inactivate the target protein, either by decreasing the protein's enzymatic activity or by blocking protein-protein interactions. Here, we provide a detailed protocol to efficiently label CRISPR-Cas complexes with a small fluorescent peptide via sortase-mediated transpeptidation. The sortase tag consists of just a few amino acids that are specifically recognized at either the N- or the C-terminus, making this strategy advantageous when the protein is part of a larger complex. Sortase is active at high ionic strength, 4°C, and with a broad range of organic fluorophores. We discuss the design, optimization, and single-molecule fluorescent imaging of CRISPR-Cas complexes on DNA curtains. Sortase-mediated transpeptidation is a versatile addition to the protein labeling toolkit.

KEYWORDS:

Cas1–Cas2; Cas3; Cascade; DNA curtains; Fluorescence; Protein labeling

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