Abstract
Reticulons are integral ER membrane proteins characterised by a reticulon homology domain comprising four transmembrane domains which results in the proteins sitting in the membrane in a W-topology. Here we report on a novel subgroup of reticulons with an extended N-terminal domain and in particular on arabidopsis reticulon 20. Using high resolution confocal microscopy we show that reticulon 20 is located in a unique punctate pattern on the ER membrane. Its closest homologue reticulon 19 labels the whole ER. Other than demonstrated for the other members of the reticulon protein family RTN20 and 19 do not display ER constriction phenotypes on over expression. We show that mutants in RTN20 or RTN19, respectively, display a significant change in sterol composition in roots indicating a role in lipid regulation. A third homologue in this family -3BETAHSD/D1- is unexpectedly localised to ER exit sites resulting in an intriguing location difference for the three proteins.
Highlights
The endoplasmic reticulum (ER) is a multifunctional organelle[1] involved in a plethora of aspects of plant life
The reticulon protein family in Arabidopsis thaliana consists of 21 members which group according to structural organisation of the functional domains, with those proteins mainly consisting of the reticulon homology domain (RTN1-16) grouping together but clearly differentiated from the reticulons with an additional N-terminal domain (RTN17-21) (Fig. 1)
RTN20 tagged to the yellow fluorescent protein was transiently expressed in tobacco leaf epidermal cells using Agrobacterium-mediated transformation (Fig. 2A)
Summary
The endoplasmic reticulum (ER) is a multifunctional organelle[1] involved in a plethora of aspects of plant life. The plant cortical ER network has been shown to play numerous roles in protein trafficking[1,3] and pathogen responses[4] It is a highly dynamic organelle and previous studies have demonstrated a possible link between ER structure and function within different cell- and tissue-types[5,6]. A variety of ER movements have been characterised, including growth and shrinkage of tubules, rearrangement of the polygonal network[2], movement of the membrane surface[7], and the conversion between cisternal and tubular ER1 These distinct movements which appear dependent on the acto/myosin system[2] and the possibly significant link between structure and function, makes these processes important to understand. Science and Technology Facilities Council (STFC) Rutherford Appleton Laboratory, Research Complex at Harwell, Didcot, OX11 0QX, United Kingdom. 5Present address: J.U.: The Sainsbury Laboratory, Norwich, United Kingdom
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