Ultrastructural characterization of endoplasmic reticulum--Golgi transport containers (EGTC)

Recent observations made in live cells expressing green fluorescent protein (GFP)-tagged cargo markers have demonstrated the existence of large, mobile transport intermediates linking peripheral ER exit sites (ERES) to the perinuclear Golgi. Using a procedure of rapid ethane freezing, we examined ultrastructurally the intermediates involved in ER-Golgi transport of the vesicular stomatitis virus (VSV) G protein. When released at the permissive temperature of 32 degrees C, VSVG is first found to be concentrated in pleiomorphic, membrane-bound structures (of about 0.4 to 1 microm in diameter) with extensive budding profiles. These structures are devoid of COPII components and Golgi markers, but are enriched in COPI, the retrograde cargo ERGIC53, and the tethering protein p115. The structures appear to be able to undergo fusion with the Golgi stack and are tentatively referred to as ER-Golgi transport containers, or EGTCs. VSVG protein exiting the ERES at 15 degrees C is first found in clusters or strings of COPII-containing small vesicles, and morphological analysis indicates that these clusters and strings of COPII vesicles may coalesce by homotypic fusion to form the EGTCs. Together with the large transport containers mediating transport from the trans-Golgi network to the plasma membrane, EGTCs represents an emerging class of large membranous structures mediating anterograde transport between the major stations of the exocytic pathway.