IQ67 DOMAIN proteins coordinate cell division.
IPB cell biologists, together with partners from Tübingen, have taken a closer look at cell division in Arabidopsis. They found that the formation of the premitotic division plane is influenced by proteins of the IQ67 DOMAIN (IQD) family. Their study was recently published in Nature Plants. IQD proteins are calmodulin-binding factors that are key regulators of numerous growth and developmental processes. In Arabidopsis, the protein family consists of 33 members. For several years now, their functional elucidation has been the declared goal of the Cellular Coordination group headed by Katharina Bürstenbinder. Initial localization studies suggested a role of the IQDs in the organization of the cytoskeleton; the exact function of the proteins, however, has so far remained elusive.
By in silico expression analyses, the Halle scientists now characterized three closely related members of the Arabidopsis IQD family: IQD6, IQD7 and IQD8. All three proteins were detected in dividing tissue of the root tip. Mutants of the three genes as well as their double and triple mutants did not show macroscopic differences compared to wild-type plants. Microscopically, however, increased frequencies of irregularly divided cells with oblique cell walls were found, especially in the iqd8 mutants. The scientists concluded that IQD8 plays a predominant role in controlling the division plane, while IQD6 and IQD7 may act in a rendundant manner.
Within cells, IQD8 localized to interphase and mitotic microtubule arrays and the cell cortex in proximity of the cortical division zone and cell plate. During cell division, this close IQD-microtubule association was already evident in preprophase - the preliminary stage of mitosis in which the microtubules arrange into a dense ring surroundingd the nucleus, that forms the so-called preprophase band. After nuclear envelope breakdown, preprophase band microtubules disassemble, and the future division plane remains occupied by molecular markers that define the cortical division zone. At the onset of cytokinesis, microtubules reorganize to form the phragmoplast, which expands centrifugally from the center of the mother cell to guide formation of the new cell plate between the nascent daughter cells. The scientists found that IQD8 localizes to both microtubules at the preprophase band and at the phragmoplast - but not at the spindle apparatus. In iqd678 triple mutants, microtubule assembly into the preprophase band was either strongly reduced or completely abolished, and phragmoplast morphology was disturbed, as indicated by a higher number of wavy and curved phragmoplasts compared to dividing wild-type cells.
The researchers identified the PHRAGMOPLAST ORIENTING KINESIN (POK) proteins and the PLECKSTRIN HOMOLOGY GTPase ACTIVATING (PHGAP) proteins as the novel interaction partners of IQDs. POKs are core components of cortical division zone setup that colocalize with the preprophase band during prophase and remain at the cortical division zone throughout mitosis. They are essential for the recruitment and retention of other cytokinesis-specific proteins, such as PHGAPs. Based on these findings, the cell biologists postulate: IQD6-8 proteins coordinate the formation of the division plane by facilitating preprophase band formation and the establishment of the cortical division zone by recruiting other components and by promoting the assembly of macromolecular complexes positioning of the cell division zone.
Original Publication:
Pratibha Kumari, Pradeep Dahiya, Pantelis Livanos, Luise Zergiebel, Malte Kölling, Yvonne Poeschl, Gina Stamm, Arvid Hermann, Steffen Abel, Sabine Müller & Katharina Bürstenbinder IQ67 DOMAIN proteins facilitate preprophase band formation and division-plane orientation. Nat. Plants (2021). https://doi.org/10.1038/s41477-021-00923-z
These findings provide essentiell new insights into the processes underlying the regulation of cell division and division plane determination, as has been highlighted in an accompanying News & Views article by cell biologist Andrei Smertenko. https://www.nature.com/articles/s41477-021-00939-5