Identification of ligand-target pairs from combined libraries of small molecules and unpurified protein targets in cell lysates.
We describe the development and validation of interaction determination using unpurified proteins (IDUP), a method that selectively amplifies DNA sequences identifying ligand+target pairs from a mixture of DNA-linked small molecules and unpurified protein targets in cell lysates. By operating in cell lysates, IDUP preserves native post-translational modifications and interactions with endogenous binding partners, thereby enabling the study of difficult-to-purify targets and increasing the potential biological relevance of detected interactions compared with methods that require purified proteins. In IDUP, target proteins are associated with DNA oligonucleotide tags either non-covalently using a DNA-linked antibody or covalently using a SNAP-tag. Ligand-target binding promotes hybridization of a self-priming hairpin that is extended by a DNA polymerase to create a DNA strand that contains sequences identifying both the target and its ligand. These sequences encoding ligand+target pairs are selectively amplified by PCR and revealed by high-throughput DNA sequencing. IDUP can respond to the effect of affinity-modulating adaptor proteins in cell lysates that would be absent in ligand screening or selection methods using a purified protein target. This capability was exemplified by the 100-fold amplification of DNA sequences encoding FRB+rapamycin or FKBP+rapamycin in samples overexpressing both FRB and FKBP (FRB·rapamycin+FKBP, Kd ≈ 100 fM; FKBP·rapamycin+FRB, Kd = 12 nM). In contrast, these sequences were amplified 10-fold less efficiently in samples overexpressing either FRB or FKBP alone (rapamycin+FKBP, Kd ≈ 0.2 nM; rapamcyin+FRB, Kd = 26 μM). Finally, IDUP was used to process a model library of DNA-linked small molecules and a model library of cell lysates expressing SNAP-target fusions combined in a single sample. In this library×library experiment, IDUP resulted in enrichment of sequences corresponding to five known ligand+target pairs ranging in binding affinity from Kd = 0.2 nM to 3.2 μM out of 67,858 possible combinations, with no false positive signals enriched to the same extent as that of any of the bona fide ligand+target pairs.
Department of Chemistry and Chemical Biology and Howard Hughes Medical Institute, Harvard University , 12 Oxford Street, Cambridge, Massachusetts 02138, United States.
Lynn M
LM
McGregor
Tara
T
Jain
David R
DR
Liu
D000906:Antibodies / Q000737:chemistry*; D001710:Biotin / Q000031:analogs & derivatives / Q000737:chemistry; D004247:DNA / Q000737:chemistry*; D006367:HeLa Cells; D006801:Humans; D008024:Ligands; D011506:Proteins / Q000737:chemistry*; D060888:Real-Time Polymerase Chain Reaction; D054852:Small Molecule Libraries / Q000737:chemistry*; D019809:Streptavidin / Q000737:chemistry
D000906:Antibodies; D008024:Ligands; D011506:Proteins; D054852:Small Molecule Libraries; D001710:Biotin; C004749:desthiobiotin; D004247:DNA; D019809:Streptavidin