A MORN-repeat protein is a dynamic component of theToxoplasma gondiicell division apparatus

Abstract

Apicomplexan parasites divide and replicate through a complex process of internal budding. Daughter cells are preformed within the mother on a cytoskeletal scaffold, endowed with a set of organelles whereby in the final stages the mother disintegrates and is recycled in the emerging daughters. How the cytoskeleton and the various endomembrane systems interact in this dynamic process remains poorly understood at the molecular level. Through a random YFP fusion screen we have identified two Toxoplasma gondii proteins carrying multiple membrane occupation and recognition nexus (MORN) motifs. MORN1 is highly conserved among apicomplexans. MORN1 specifically localizes to ring structures at the apical and posterior end of the inner membrane complex and to the centrocone, a specialized nuclear structure that organizes the mitotic spindle. Time-lapse imaging of tagged MORN1 revealed that these structures are highly dynamic and appear to play a role in nuclear division and daughter cell budding. Overexpression of MORN1 resulted in severe but specific defects in nuclear segregation and daughter cell formation. We hypothesize that MORN1 functions as a linker protein between certain membrane regions and the parasite's cytoskeleton. Our initial biochemical analysis is consistent with this model. Whereas recombinant MORN1 produced in bacteria is soluble, in the parasite MORN1 was associated with the cytoskeleton after detergent extraction.

Unfolding the secrets of coral–algal symbiosis

“…An interesting characteristic of these genes is that they encompass multiple copies of the membrane occupation and recognition nexus (MORN) repeat. Proteins containing MORN repeats could be involved in cell division, as in the parasite Toxoplasma gondii, where they are located in the cell division apparatus and are proposed to play a role in the cytoskeleton interaction between the parasite and host (Gubbels et al, 2006;Takeshima et al, 2000). In plants, the MORN repeat motif has been discovered in the ARC3 gene that is important for the replication of chloroplasts (Shimada et al, 2004).…”

Toxoplasma gondii aspartic protease 1 is not essential in tachyzoites

“…TgDLC localized to a part of the IMC described as the apical cap in the two strains (data not shown). TgASP1 accumulates in the nascent daughter cells in close proximity to the TgMORN1 ring at the base of forming daughter cell IMCs (Gubbels et al, 2006;Heaslip et al, 2010). TgMORN1 has recently been shown to be the dynamic key organizer for the basal complex (Heaslip et al, 2010) and the redistribution of the protein during parasite division does not appear to be altered in asp1-ko (Fig.…”

“…Transient tranfections were done with pPhil1-YFP (Gilk et al, 2006), pDLC-EGFP , pMORN-EGFP (Gubbels et al, 2006), and pGRASP-YFP (Pelletier et al, 2002) in RH and Tgasp1-strains. Co-localizations were done using anti-TgGAP45 antibodies as described previously with goat-anti-rabbit IgG conjugated Alexa-Fluor-594 as secondary antibodies.…”

Giant Marseillevirus highlights the role of amoebae as a melting pot in emergence of chimeric microorganisms

“…The largest family consists of 20 proteins containing bacteriallike membrane occupation and recognition nexus (MORN) repeat domains that typically mediate membrane-membrane or membrane-cytoskeleton interactions (12). Marseillevirus is unusually rich in serine/threonine protein kinases, with two distinct clusters of 11 and three kinases, respectively, and a unique kinase shared by Marseillevirus, Iridoviruses, and Ascoviruses (Tables S3 and S4).…”