mycARNO3 and ARNO overexpression inhibit intracellular transport of SEAP. HeLa cells were cotransfected with pGEM-SEAP and pGEM-mycARNO3 or pGEM-ARNO or pGEM-1 plasmids. Cells were pulsed for 10 min, chased for 2 h, and solubilized. SEAP was immunoprecipitated and treated or not with endoH. m, the mature form of SEAP (endoH-resistant); i, the immature form still associated with the endoplasmic reticulum; s, the endoH-digested immature form, and s′, the endoH-digested form detected only in ARNO3- and ARNO-expressing cells, which probably is O-glycosylated.