MORC Proteins: Novel Players in Plant and Animal Health

Abstract

Microrchidia (MORC) proteins comprise a family of proteins that have been identified in prokaryotes and eukaryotes. They are defined by two hallmark domains: a GHKL-type ATPase and an S5 fold. MORC proteins in plants were first discovered via a genetic screen for Arabidopsis mutants compromised for resistance to a viral pathogen. Subsequent studies expanded their role in plant immunity and revealed their involvement in gene silencing and transposable element repression. Emerging data suggest that MORC proteins also participate in pathogen-induced chromatin remodeling and epigenetic gene regulation. In addition, biochemical analyses recently demonstrated that plant MORCs have topoisomerase II (topo II)-like DNA modifying activities that may be important for their function. Interestingly, animal MORC proteins exhibit many parallels with their plant counterparts, as they have been implicated in disease development and gene silencing. In addition, human MORCs, like plant MORCs, bind salicylic acid and this inhibits some of their topo II-like activities. In this review, we will focus primarily on plant MORCs, although relevant comparisons with animal MORCs will be provided.

Keywords: RNA interference (RNAi); RNA-directed DNA methylation; human MORCs; immunity; pathogen; plant MORCs; transcriptional gene silencing.

Figure 1

Schematic of MORC architecture. Domain organization of MORC family members from Arabidopsis thaliana (At), Hordeum vulgare (Hv), and Homo sapiens (Hs) are shown. In plant and animal MORC proteins, the N-terminal region contains a highly conserved GHKL-ATPase domain combined with an S5 fold domain. These domains are connected via an unstructured region or linker (L1). In addition to the N-terminal GHKL-ATPase module, many plant MORC proteins contain a coiled-coil (CC) domain at their C-terminal region that is connected via a linker (L2) to the S5 fold domain. Other unstructured regions or linkers connecting different structured domains are depicted as linkers 3–5 (L3–L5). Many animal MORC proteins and some plant MORC proteins also carry a zinc-finger CW domain in the C-terminal region of the protein. Some animal, but not plant, MORC proteins carry an additional CC domain between the GHKL-ATPase domain and the S5 fold domain. Protein domains were drawn using MyDomains—Image creator (http://prosite.expasy.org/mydomains/). Note all protein domain structures have been drawn to scale except AtGMI1 and HsSMCHD1.

Figure 2

Schematic model of MORC involvement in RdDM. DNA methylation in Arabidopsis is regulated via the RNA-directed DNA methylation (RdDM) pathway. H3K9me2 methylation marks recruit Pol IV to its genomic loci via SHH1 whereas CLSY1 facilitates Pol IV transcription. The single-stranded Pol IV transcripts are converted to dsRNA by RDR2 and subsequently processed by DCL3 in 24-nt siRNAs. Before loading of these siRNAs onto AGO4 (and/or AGO6/9) and export to the cytoplasm they are stabilized by HEN1-mediated 3′ end methylation. AGO4-siRNA complexes are then reimported to the nucleus, where they target in a sequence-specific manner nascent Pol V scaffold transcripts to recruit DRM2 (cytosine-5-methyltransferase), which catalyzes de novo methylation at a certain loci. Pol V recruitment to its genomic loci is mediated by the DDR complex. AtMORC6 together with AtMORC1 and AtMORC2 form a second complex (MORC complex), that is thought to be required for the recruitment of Pol V to silenced loci. Therefore, AtMORC6 is interacting with DMS3, a member of the DDR complex, probably to provide the missing ATPase activity for DMS3. Furthermore, AtMORC6, AtMORC1, and AtMORC2 interact with SUVH2 and/or SUVH9 that act as adaptors to bind methylated DNA and the DDR complex and, in conjunction with AtMORC proteins, recruit Pol V. Pol V transcripts thereby serve as scaffolds for the assembly of the IDN2-IDP and SWI/SNF chromatin remodeling complexes that adjust nucleosome positioning. The AtMORC proteins were found to directly interact with IDN2 and/or various subunits of the SWI/SNF complex, thus establishing positioned nucleosomes to effect silencing. Building on this model, it was proposed that binding of methylated DNA by SUVH2 and SUVH9 initially mediates RdDM (via recruitment of the DDR complex and MORC complex) and subsequently facilitates recruitment of a MORC-IDN2-SWI/SNF complex that alters chromatin structure, potentially by positioning nucleosomes at the targeted locus, thereby reinforcing TGS (see text for more information).