Membrane-Based Affinity Purification to Identify Target Proteins of a Small-Molecule Drug.

Identifying the target proteins of small-molecule drug candidates is important for determining their molecular mechanisms of action. Porous membranes derivatized with such small molecules may provide an attractive target-identification platform due to a high protein-capture efficiency during flow through membrane pores. This work employs carbonic anhydrase II (CAII) binding to immobilized 4-(2-aminoethyl)benzenesulfonamide (AEBSA) to examine the efficiency and selectivity of affinity capture in modified membranes. Selective elution of captured protein, tryptic digestion, tandem mass spectrometry analysis, and label-free quantification (LFQ) identify CAII as the dominant AEBSA target in diluted serum or cell lysate. CAII identification relies on determining the ratio of protein LFQ intensities in sample and control experiments, where free AEBSA added to the control loading solution limits CAII capture. Global proteomics shows that the spiked CAII is the only protein with a log2 ratio consistently >2, and the detection limit for CAII identification is 0.004 wt % of the total protein in 1:4 diluted human serum or 0.024 wt % of the total protein from breast cancer cell lysates. The same approach also identifies native CAII in human kidney cell lysate as an AEBSA target. Comparison of affinity capture using membranes, Affi-Gel 10 resin or M-270 Dynabeads derivatized with AEBSA suggests that only membranes allow identification of low-abundance CAII as a target.