In Arabidopsis, orthologs of the human mRNA export adaptors ALY and UIF are diversified with the genome encoding four ALY family members and two UIF-like factors. In this study four ALY and two UIEF proteins were characterised as mRNA export factors and seven candidates that co-purified with ALY and UIEF proteins in affinity purification experiments were further analysed as possible mRNA export receptors.
ALY and UIEF proteins directly interacted with the TREX subunit UAP56 demonstrating that ALY and UIEF proteins are recruited to TREX by an interaction with the RNA helicase UAP56. Plants lacking all four ALY proteins were severely affected in plant growth and development accompanied by a general mRNA export defect demonstrating that ALY proteins function as mRNA export factors. uief single and double mutant plants displayed defects in plant growth and development and showed a moderate mRNA export block. Since plants lacking four ALY and two UIEF proteins displayed more pronounced plant growth and development defects and showed a stronger mRNA export block than 4xaly plants, also the UIEF proteins could be described as mRNA export factors. The observation that aly single and double mutant plants exhibited no obvious phenotype but 4xaly displayed a pleiotropic phenotype indicated that ALY proteins can function redundantly. Mutants lacking three ALY proteins displayed different phenotypes that could be clearly distinguished from wildtype and 4xaly plants what demonstrated that besides functional redundancy different ALYs also have specific functions. RNA sequencing of nuclei and whole cells showed that 4xaly is a general mRNA export mutant and additional transcriptomic analysis of aly and uief mutants demonstrated that ALY proteins regulate the mRNA export of a variety of transcripts. The influence of UIEF proteins on mRNA export on the other hand is limited to only a subset of transcripts. Transcriptomic analysis and export assays of 3xaly mutant plants further revealed that the different phenotypes of 3xaly and 4xaly mutant plants are rather caused by defects in mRNA export of specific transcripts or subsets of transcripts than by differences in a bulk mRNA export block.
Proteomic analysis of proteins co-purifying with the four ALY and two UIEF export factors identified seven candidates that share some features with the human heterodimeric export receptor. Fusion proteins of the seven candidates were detected in the cytoplasm of cells and an enriched signal could be detected around the nuclear envelope when candidate fusion proteins were expressed in tobacco leaf cells. Plants lacking several receptor candidates did not show mRNA export defects indicating that plant mRNA receptors could be highly diversified, but additional studies are needed to characterize the seven mRNA receptor candidates.