Decisive Molecular Interactions in the early secretory pathway of plant cells
Final Report Abstract
This research project centred on two aspects of the early secretory pathway in plants. The first subproject dealt with the identification of ER import sites (ERIS) and their relationship to ER export sites (ERES). For this invesitigation several new fluorescent markers were generated: the 6 kDA VP-CFP as a COPII marker, TIP20-YFP as an ER- tethering factor, the Q- SNARE SYP72-GFP as an ERIS marker. Firstly, we established that fluorescent COPII markers are not visible at the surface of the ER, but instead locate between the ER and the cis-Golgi representing the pre-Golgi accumulation of COPII vesicles. Secondly, when Golgi stacks are immobilized the fluorescent signals of all these markers colocalize with standard Golgi markers. Thirdly, during Golgi movement, the ER-tethering factor TIP20 remains colocalized with the overlying Golgi stack. These data have enabled us to propose a new model for ER-Golgi transport in higher plants. We suggest that whereas COPII vesicle release from the ER is a continuous process being triggered by an approaching Golgi stack, fusion with the cis- Golgi cisternae is delayed until the Golgi stack is immobile. Similarly, COPI vesicle formation is a continuous process, and the retrograde carriers only fuse with the ER when the Golgi stack is stationary. Hence we have bidirectional vesicle transport system exclusively restricted to a mobile ER-Golgi transport unit. The second subproject was focussed on the p24 family of type 1 membrane proteins, which are one of the most interesting groups of proteins involved in regulating the structure and function of the organelles of the secretory pathway. p24 proteins can be classified into four different sub-families by sequence homology, named p24α, p24ß, p24γ and p24δ. Whereas animals and fungi have representatives in all four sub-families, plants have only members of the p24δ (9 in Arabidopsis) and the p24ß (2 in Arabidopsis) subfamilies. We have shown that all members of the p24δ sub-family in Arabidopsis locate to the ER whereas the p24ß sub-family are found in the Golgi apparatus under steady state conditions. By a combination of experiments using transient expression, knock-out mutants and co-immunoprecipitation we propose that Arabidopsis p24 proteins form different heteromeric complexes for their coupled trafficking at the ER- Golgi interface. We also provide the first evidence for a role of p24d5 in retrograde Golgi-to-ER transport of the KDEL-receptor ERD2 in plants.
Publications
- (2010) Sorting of plant vacuolar proteins is initated in the ER. Plant J 62: 601-614
Niemes S, Labs M, Scheuring D, Krueger F, Langhans M, Robinson DG, Pimpl P
- (2011) Is the 6 kDa tobacco etch viral protein a bona fide ERES marker? J Exp Bot. 62: 5013-5023
Lerich A, Langhans M, Sturm S, Robinson DG
- (2012) Coupled transport of Arabidopsis p24 proteins at the ER-Golgi interface. J Exp Bot 63: 4243- 4261
Montezinos JC, Sturm S, Langhans M, Hillmer S, Marcote MJ, Robinson DG, Aniento F
(See online at https://doi.org/10.1093/jxb/ers112) - (2012) ER import sites and their relationship to ER exit sites: a new model for bidirectional ER-Golgi transport in higher plants. Frontiers in Plant Cell Biol
Lerich A, Hillmer S, Langhans M, Scheuring D, van Bentum P, Robinson DG
(See online at https://doi.org/10.3389/fpls.2012.00143) - (2012) ERES (ER exit sites) and the “Secretory Unit Concept”. J. Microsc. 247: 48-59
Langhans M, Meckel T, Kress A, Lerich A, Robinson, DG
(See online at https://doi.org/10.1111/j.1365-2818.2011.03597.x) - (2012/13) Putative p24 complexes in Arabidopsis contain members of the delta and beta subfamilies and cycle in the early secretory pathway. J Exp Bot
Montezinos JC, Langhans M, Sturm S, Hillmer S, Aniento F, Robinson DG, Marcote MJ
(See online at https://doi.org/10.1093/jxb/ert157)
