Compositions and methods for assessing protein function are provided. A modified SpyCatcher protein that can include a tag is provided. Modified SpyCatcher proteins linked to a protein of interest, such as a nuclease are also provided. The methods include contacting a SpyCatcher protein and a SpyTagged protein to form a complex that may further include a protein of interest, one or more nucleic acids, and/or a nuclease. The methods can be used to purify a protein of interest or identify or target a protein binding site in a nucleic acid.