Determining the activity of the transporter associated with antigen processing in the compartments of the secretory pathway.

Peptide-receptive MHC class I molecules and the TAP (transporter associated with antigen processing) peptide transporter are known to leave the ER and cycle through the cis side of the Golgi apparatus. The amount, and the extent of the activity, of TAP in post-ER compartments is likely to vary between different cell types. Here we describe a convenient microscopic assay to determine it.