OPENER Is a Nuclear Envelope and Mitochondria Localized Protein Required for Cell Cycle Progression in Arabidopsis
Plant Cell 2019, 31(7):1446-1465
Wang W, Zhang X, Niittyla T

Abstract
Currently one third of the proteins encoded by the Arabidopsis thaliana genome are of unknown function. Some of these unknown proteins are likely to be involved in uncharacterized vital biological processes. Evolutionarily-conserved single copy genes in flowering plants have been shown to be enriched in essential housekeeping functions. This together with publicly available gene expression data allows for a focused search for uncharacterized essential genes. Here we identify an essential single copy gene called OPENER (OPNR) in Arabidopsis thaliana. We show that OPNR is predominantly expressed in actively dividing cells and performs essential functions in seed developmentand root meristem maintenance. Cell cycle tracking using 5-ethynyl-2'-deoxyuridine (EdU) staining and fluorescent cell cycle markers together with the increased size of nucleolus and nucleus in opnr mutants indicate that OPNR is required for cell cycle progression through the S or G2 phases. Intriguingly, OPNR localizes to the nuclear envelope and mitochondria. Furthermore, the nuclear envelope localization of OPNR is dependent on its interaction with nuclear inner membrane Sad1/UNC-84 (SUN) domain proteins SUN1 and SUN2. Taken together our results open a line of investigation into an evolutionarily-conserved essential cellular process occurring in both the nuclear envelopes and mitochondria of dividing cells.

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