Ultrastructural analysis of transitional endoplasmic reticulum and pre-Golgi intermediates: a highway for cars and trucks

Abstract

Cargo selection and export from the endoplasmic reticulum occurs at specialized sites in cells. Export complexes consist of transitional elements of the endoplasmic reticulum and pre-Golgi intermediates. It is generally assumed that 60 to 80 nm initially COPII-coated vesicles derived from the transitional endoplasmic reticulum are the main carriers for transport of cargo to the Golgi apparatus. We have analyzed on serial ultrathin sections the transitional endoplasmic reticulum and pre-Golgi intermediates of beta cells of islets of Langerhans in mouse pancreas. In addition to Golgi-associated complexes, others were observed in the periphery of the cells or close to the nuclear envelope. Upon three-dimensional reconstruction, non-coated ribosome-free tubules with an average diameter of 115 nm (range 60-195 nm) and a length of up to 500 nm were detected in the pre-Golgi intermediates in addition to small vesiculo-tubular elements. Furthermore, evidence was found that the large tubular elements may directly arise from transitional elements of the endoplasmic reticulum. In a given cell, pre-Golgi intermediates were found to be composed solely of small vesiculo-tubular elements or additionally of tubules or solely of tubules. Immunogold labeling for proinsulin indicated that the large tubular elements contained cargo and thus appear to take part in ER-to-Golgi transport.

Glycosyltransferase-specific Golgi-targeting Mechanisms

“…The COPII-independent ERto-Golgi trafficking of C2GnT-M and C1GalT1 was further supported by unaltered COPII distribution after KD of Giantin or GM130 (data not shown). Our data fit well with the reported observations that non-golgin Golgi proteins are involved in the docking of COPII (29) and that 500-nm range tubular transport carriers are found shuttling between ER and Golgi (30). This transport mechanism requires intact microtubules and occurs independent of COPI or COPII coats (31,32).…”

Expression of mutant Ins2C96Y results in enhanced tubule formation causing enlargement of pre-Golgi intermediates of CHO cells

“…1, 2, 3, 4). Their structural organization corresponded basically to the one reported for other cell types (Bannykh and Balch 1997;Fan et al 2003;Farquhar and Hauri 1997;Hauri et al 2000;Palade 1975;Saraste and Svensson 1991;Zuber et al 2004). Although the vesiculo-tubular clusters could be followed through more than ten serial sections, particularly in CHO Ins2 C96Y cells, the budding zone of the transitional ER was only observed in one or two sections.…”

“…A prominent feature of the expanded pre-Golgi intermediates were tubules with an average diameter of 102 nm and a length of up to 500 nm. Tubules of the same dimensions existed in the pre-Golgi intermediates of b-cells of control mice (Fan et al 2003). However, their number was significantly increased in Akita mice b-cells.…”

Functional Symmetry of Endomembranes

“…The IC was originally described as large vacuoles (up to 1 m in diameter) containing cargo proteins en route from the ER to the Golgi apparatus (Saraste and Kuismanen, 1984;Kuismanen and Saraste, 1989). Subsequent studies have verified these observations (Ying et al, 2000;Horstmann et al, 2002;Fan et al, 2003;Mironov et al, 2003), but also shown that cargo proteins at the ER-Golgi boundary are localized to vesicular-tubular clusters (VTCs; Balch et al, 1994, MartinezMenarguez et al, 1999. However, the relationship between the pre-Golgi vacuoles and the VTCs has remained unclear.…”