Lima bean
agglutinin-
fluorescein 5-isothiocyanate conjugate (FluNCS-
lima bean lectin) interacts with specific receptor molecules on membranes both from the rod outer segment (ROS) of the frog retina and from S49 mouse
lymphoma cells. When [125I]-5-iodonaphthyl 1-azide (125I-INA), which freely and randomly partitions into the
lipid bilayer, is added to membranes and the
suspension is irradiated at 480 nm, the FluNCS-conjugated
lectin photosensitizes the [125I]INA but only at discrete sites. This results in the selective labeling of specific
proteins: an 88-kDa
protein on ROS membranes and a 56-kDa
protein on S49 plasma membranes. Labeling is dependent upon the interaction of the FluNCS-
lectin with glycosylated receptor sites, since N-
acetylgalactosamine, but not methyl alpha-
mannoside, blocked labeling of the 56-kDa
protein on S49 membranes. In contrast, a random labeling pattern of
membrane proteins was observed upon irradiation at 480 nm using other
fluorescein conjugates, such as FluNCS-
bovine serum albumin (FluNCS-BSA) or FluNCS-soybean
trypsin inhibitor (FluNCS-
STI), which interact with cell membranes in a nonselective manner, or with N-(fluorescein-5-thiocarbamoyl)-n-undecyclamine (FluNCS-NHC11), which is freely miscible in the
membrane lipid. Random labeling was also obtained by direct photoexcitation of [125I]INA at 314 nm, with no distinct labeling of the 88- and 56-kDa
proteins in the respective membranes. These results suggest that
protein ligands can be used to guide sensitizers to discrete receptor sites and lead to their selective labeling by photosensitized activation of [125I]INA [Raviv, Y., Salomon, Y., Gitler, C., & Bercovici, T. (1987) Proc. Natl. Acad. Sci. U.S.A. 84, 6103-6107].(ABSTRACT TRUNCATED AT 250 WORDS)