The endoplasmic reticulum (ER) contains dynamic subdomains — transitional ER (tER) sites — that are specialized for coat protein II (COPII)-vesicle production and that are distinct from the ER in terms of their function, biochemistry and morphology. Although these tER sites were described more than 25 years ago, how they are formed and maintained, and how their dynamics affect Golgi dynamics, has remained unclear. Now, though, in Nature Cell Biology, Glick and colleagues provide new insights.

The authors labelled tER sites of the budding yeast Pichia pastoris using a green fluorescent protein (GFP)–Sec13 construct (Sec13 is a COPII-coat subunit), and used three-dimensional time-lapse (4D) confocal video microscopy to monitor these tER sites. In both the mother cells and buds, they saw the apparent de novo formation of tER sites — small fluorescent spots appeared as if by magic and then increased in size. They also noted that tER sites can fuse, and that they seem to have a 'preferred' size — they shrink after fusion and expand after de novo formation.

To study how tER dynamics relate to Golgi dynamics, Glick and co-workers used Sec7 — a late Golgi protein. They overexpressed a Sec7–Discosoma red fluorescent protein (DsRed) construct, and used two-colour 4D microscopy to monitor tER sites (GFP–Sec13) and the Golgi (Sec7–DsRed) at the same time. They consistently saw that tER-site formation is tightly linked to the apparent de novo formation of Golgi structures. In addition, they saw that late Golgi elements labelled with Sec7–DsRed often move to regions of polarized growth, that is, away from the tER and towards the bud.

Glick and colleagues have shown that tER sites and Golgi structures can apparently form de novo in P. pastoris, although they note that templates could be present that are invisible in their experiments. However, despite these uncertainties, their data “can be explained by assuming that tER sites give rise to Golgi cisternae that continually mature”.