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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.

The family of reticulon proteins has been shown to be involved in a variety of functions in eukaryotic cells including tubulation of the endoplasmic reticulum (ER), formation of cell plates and primary plasmodesmata. Reticulons are integral ER membrane proteins characterised by a reticulon homology domain comprising four transmembrane domains which results in the reticulons sitting in the membrane in a W-topology. Here we report on a subgroup of reticulons with an extended N-terminal domain and in particular on arabidopsis reticulon 20. 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. We show that mutants in RTN20 or RTN19, respectively, display a significant change in sterol composition in the roots indicating a role in lipid biosynthesis or regulation. A third homologue in this family - 3BETAHSD/D1- is localised to ER exit sites resulting in an intriguing location difference for the three proteins.