Ashton Cropp, Ph.D.

Oliver Hall 3047


B.S., Western Carolina University, 1997
Ph.D, Virginia Commonwealth University, 2002
NIH Postdoctoral Fellow, The Scripps Research Institute 2002-04

Research interests

The research in the Cropp group is focused on developing tools to study protein function.  Students gain experience in synthetic chemistry, molecular biology, protein expression and purification by pursuing projects in the following two areas.

Synthetic genetic code expansion

Protein biosynthesis is normally limited by the 20 genetically encoded amino acids.  However the genetic code can be expanded by the use of orthogonal aminoacyl-tRNA synthetase/tRNA pairs that can reassign the amber stop codon, TAG.  Using directed evolution the specificity of synthetase/tRNA pairs can be evolved to accommodate new chemical amino acids that contain functionality not found in cells.  Our group has recently been interested in use of protecting groups to “disguise” unnatural amino acids such that they can be revealed after translation.  This allows one to either control biological processes with external triggers and/or hide amino acids that would normally interfere with natural protein translation.  Current projects involve the synthesis and genetic encoding of novel amino acids and the evolution of new translational components using in vitro techniques.

Ubiquitin protein chemistry

Ubiquitin (Ub) is a small, 76 amino acid protein that serves as a posttranslational modification in eukaryotes. Unlike other modifications, Ub modifications can be polymerized (poly-Ubs) at one of seven lysine residues, resulting in an array of biological signals.  Traditionally, the study of these modifications has been hampered by synthetic access to poly-Ub chains.  Using unnatural amino acid mutagenesis we have been able to direct the synthesis of all poly-Ub linkages.  Current efforts focus on using synthetic chains to decipher the role of linkage-specificity in ubiquitin signaling.

Select publications

Li Y, Reed M*, Wright HT, Cropp TA, Williams GJ. Development of Genetically Encoded Biosensors for Reporting the Methyltransferase-Dependent Biosynthesis of Semisynthetic Macrolide Antibiotics. ACS Synth Biol. 2021 doi: 10.1021/acssynbio.1c00151.

Zhou, H.,Cheung, J.W.*, Carpenter, T., Jones, S., Luong, N.H., Tran, N.C., Jacobs, S.E., Cropp, T.A., Yin, J. Enhancing the incorporation of pyrrolysyl derivatives into proteins with the methylester form of unnatural amino acids. Bioorg Med Chem Lett. 2020, 30, 126876.

Braxton CN, Quartner E, Pawloski W, Fushman D, Cropp TA. Ubiquitin chains bearing genetically encoded photo-crosslinkers enable efficient covalent capture of (poly)ubiquitin-binding domains. Biochemistry. 2019 Jan 22. doi: 10.1021/acs.biochem.8b01089.

Kasey CM, Zerrad M, Li Y, Cropp TA, Williams GJ. Development of Transcription Factor-Based Designer Macrolide Biosensors for Metabolic Engineering and Synthetic Biology. ACS Synth Biol. 2018, 7, 227-239.

Ring CM, Iqbal ES, Hacker DE, Hartman MCT, Cropp TA. Genetic incorporation of 4-fluorohistidine into peptides enables selective affinity purification. Org Biomol Chem. 2017,15, 4536-4539.

Castañeda CA, Dixon EK, Walker O, Chaturvedi A, Nakasone MA, Curtis JE, Reed MR, Krueger, S, Cropp TA, Fushman D. Linkage via K27 Bestows Ubiquitin Chains with Unique Properties among Polyubiquitins. Structure. 2016, 24, 423-36.

Knight WA, Cropp TA, Genetic encoding of the post-translational modification 2-hydroxyisobutyryl-lysine. Org Biomol Chem. 2015, 13, 6479-6481.

Liu, J. and Cropp, T.A. Rational protein sequence diversification by multi-codon scanning mutagenesis. Meth. Mol. Biol., 2013, 978, 217-228.

Liu, J. and Cropp, T.A. A method for multi-codon scanning mutagenesis of proteins based on asymmetric transposons. Protein Eng. Des. Selec., 2012, 25, 67-72.

Castaneda, C.; Liu, J; Chaturvedi, A.; Nowicka, U.; Cropp, T.A.; Fushman, D. Non-enzymatic assembly of natural ubiquitin chains of any desired length, linkage composition, and isotopic labeling scheme., J. Am. Chem. Soc., 2011, 133, 17855-17868.

Gardner, L.; Zou, Y.; Mara, A.; Cropp, T.A.; Deiters, A. Photochemical control of bacterial signal processing using a light-activated erythromycin., Mol. Biosys., 2011, 7, 2554-2557.

Liu, J. and Cropp, T.A. Experimental methods for scanning unnatural amino acid mutagenesis. Meth. Mol. Biol., 2011, 794, 187-197.

Castañeda, C.A.; Liu, J.; Kashyap, T.R.; Singh, R.K.; Fushman, D.; Cropp, T.A. Controlled enzymatic synthesis of natural-linkage, defined-length polyubiquitin chains using lysines with removable protecting groups. Chem Comm., 2011, 47, 2026-2028.

Liu, J.; Castañeda, C.A.; Wilkins, B.J.; Fushman, D.; Cropp, T.A.  Condensed E. coli cultures for highly efficient production of proteins containing unnatural amino acids. Bioorg. Med. Chem. Lett., 2010, 20, 5613-5616.

Wilkins, B.J.; Young, D.D.; Marionni, S.; Liu, J.; Deiters, A.; Cropp, T.A. Site-specific incorporation of fluorotyrosines into proteins by photochemical disguise. Biochemistry, 2010, 49, 1557-1559.

Wilkins, B.J.; Yang, X.; Cropp, T.A. Photochemical activation of FlAsH labeling of proteins. Bioorg. Med. Chem. Lett., 2009, 19, 4296-4298.

Daggett, K.A.; Layer, M.; Cropp, T.A. A general method for scanning unnatural amino acid mutagenesis. ACS Chemical Biology. 2009, 4, 109–113.

Zheng, J.; Sagar, V; Smolinsky, A.; Bourke, C.; LaRonde-Leblanc, N.; Cropp, T.A., Structure and function of the macrolide biosensor, MphR(A), with and without erythromycin.  J. Mol. Biol., 2009, 387,1250-1260.

Wilkins, B.J.; Daggett, K.A.; Cropp, T.A. Peptide mass fingerprinting using isotopically encoded photo-crosslinking amino acids.  Mol. Biosys., 2008, 4, 934-936.