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

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.