Intracellular retention of proNE is augmented by CD63. COS cells transfected with NE, GFP-CD63, NE plus GFP-CD63, and NE plus GFP-CD63ΔLEL (lacking the large extracellular loop) were subject to biosynthetic radiolabeling for 0.5 hours followed by chase of the radiolabel for 1.5 and 4 hours. Cells were extracted in RIPA and immunoprecipitated with anti-NE or anti-cproNE followed by SDS-PAGE and fluorography. Both anti-NE and anti-cproNE detect proNE in cells expressing NE and NE plus GFP-CD63 in a similar manner in both cells and medium. Cells transfected with GFP-CD63 did not show any signal after IP with anti-NE or anti-cproNE. Coexpression of NE and GFP-CD63 but not NE and GFP-CD63ΔLEL led to increased intracellular retention of proNE.