Plant mobile domain proteins MAIN and MAIL1 interact with the phosphatase PP7L to regulate gene expression and silence transposable elements.

Transposable elements (TEs) are DNA repeats that must remain silenced to ensure cell integrity. Several epigenetic pathways including DNA methylation and histone modifications are involved in the silencing of TEs and in the regulation of gene expression. In Arabidopsis, additional pathways such as the Microrchidia 1 (MORC1)/MORC6 pathway are required for TE silencing. Recently, TE-derived plant mobile domain (PMD) proteins have been related to TE silencing but also genome stability, and control of developmental processes. Thus, it has been proposed that PMDs could be involved in the regulation of gene transcription. Using a forward genetic screen in Arabidopsis, we found that the PMD protein MAINTENANCE OF MERISTEMS (MAIN) act synergistically and redundantly to DNA methylation to silence TEs. We then showed that MAIN and its close homolog MAIN-LIKE 1 (MAIL1) interact together, but also with the phosphoprotein phosphatase (PPP) PP7-like (PP7L). Remarkably, mutants for MAIN, MAIL1 and PP7L display similar developmental phenotypes, and share common subsets of upregulated TEs and misregulated genes. Among the misregulated genes, we could identify discrete DNA motifs, suggesting that these genes are transcriptional regulated through similar mechanisms. Finally, phylogenetic analyses of PMD and PP7 domains among the angiosperm lineage suggest neo-association processes between the two protein domains to potentially generate new protein function. We propose that, through this interaction, the PMD and PP7 domains may constitute a functional protein module required for the proper expression of genes and silencing of TEs.