K5 mediates proteasomal destruction of newly synthesized, ER-resident CD31. (A) AdTet or AdK5-transduced E-DMVECs were metabolically labeled for 45 minutes (P) at 24 hours after infection, and the label was chased for the indicated times. Immunoprecipitates were EndoH treated. (B) Cells were infected and labeled as in panel A except that lactacystin was added prior to labeling and immunoprecipitation and EndoH treatment. (C) Surface expression of MHC I and CD31 was simultaneously monitored by flow cytometry in E-DMVECs transduced by AdTet or AdK5 (unshaded) with or without lactacystin treatment. CD31 or MHC I expression on uninfected E-DMVECs is shaded.