Labeling of the active subunit of cholera toxin from within the membrane bilayer.

Using the photoreactive glycolipid probe 12-(4-azido-2-nitrophenoxy)-stearoylglucosamine-[1-¹⁴C], we have effected the radiolabeling of the active A₁ subunit of cholera toxin from within the membrane bilayer. The membrane employed as a target was the envelope of Newcastle disease virus which contained the photoreactive probe. Radiolabeling of the A₁ subunit occurred after cholera toxin and virus were incubated together for 15 min at 37° and then irradiated at 366 nm for 1 min. Labeling of A₁ did not occur when cholera toxin was irradiated in a solution of probe without virus or when the 15 min incubation with virus was performed at 0° instead of at 37°.