Photoaffinity labeling of proteins using diazirine-functionalized peptides. Panel A) Labeling of rFTase: Lane 1: No diazirine (19 instead of 20a), + UV; Lane 2: Probe (20a), – UV; Lane 3: Probe (20a), + UV; Lane 4: Probe (20a) + UV, + competitor (16). Panel B) Labeling of SmgGDS isoforms: Lane 1: SmgGDS558, No diazirine (19 instead of 20a), + UV; Lane 2: SmgGDS558, Probe (20a), – UV; Lane 3: SmgGDS558, Probe (20a), + UV; Lane 4: SmgGDS607 No diazirine (19 instead of 20a), + UV; Lane 5: SmgGDS607, Probe (20a), – UV; Lane 6: SmgGDS607, Probe (20a), + UV. Note all reactions with SmgGDS proteins also contained yFTase that was used to enzymatically prepare 20a from 19 and 1 prior to photolysis. In Panel A, Lane 1, and Panel B, Lanes 1 and 4, diazirine 1 was omitted to prevent the formation of 20a from 19. Compound 1 was present in all other samples.