Determining protein transport to the plasma membrane.

Many integral membrane proteins synthesized in the endoplasmic reticulum ultimately arrive at the cell surface to contact the cell environment. During transit through the Golgi and trans-Golgi network, proteins acquire post-translational modifications that can be used to track the appearance of such modified proteins at the cell surface. Cellular proteins can be treated with enzymes-e.g., sialidase or protease-or antibodies, or biotinylated to identify molecules that have reached the cell surface. Some proteins first enter the endocytic pathway before appearing at the cell surface; this is detected by treating the cells at 4 degrees and 37 degrees C. Analysis of the number of sialic acids on proteins of cells treated at 4 degrees C identifies proteins resident at the cell surface, while cells treated at 37 degrees C internalize the sialidase, which can then act with proteins in the endocytic compartments.