Photoaffinity labeling of dog pancreas microsomes with 8-azido-ATP inhibits association of nascent preprolactin with the signal sequence receptor complex

Transport of bovine preprolactin into dog pancreas microsomes involves a microsomal protein which is sensitive to photoaffinity labeling with azido-ATP and which is distinct from the ATP-binding protein, immunoglobulin heavy chain binding protein. Here we addressed the question of what stage of preprolactin transport is affected. Thus a nascent presecretory protein which is related to preprolactin, termed ppl-86mer, was employed. Here we show that the nascent preprolactin did not become associated with the α-subunit of the signal sequence receptor complex after photoaffinity labeling of microsomes with azido-ATP. Therefore, we conclude that the microsomal protein which is sensitive to photoaffinity labeling with azido-ATP acts prior to the signal sequence receptor complex.