Assembly and dynamics of macromolecular protein complexes at the membrane-microtubule nexus during cell division

• Applicant: Professorin Dr. Katharina Bürstenbinder
• Subject Area: Plant Cell and Developmental Biology
• Term: since 2018
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 396136985

Precise spatio-temporal regulation of cell division is fundamental for plant growth and morphogenesis. The plant cytoskeleton, comprised of actin filaments and microtubules, plays essential roles during cell division. During G2 to M transition, cortical interphase microtubules reorganize into a broad band, termed preprophase band (PPB) that encircles the cell at the future division site. The PPB disassembles at late prophase. Its positional information is maintained throughout cell division by a polarized membrane domain, referred to as cortical division zone (CDZ). The formation of a new cell wall is initiated at the cell center after separation of chromosomes by the spindle apparatus. It is facilitated by the phragmoplast, a bipolar array of microtubules that serves as scaffold for vesicle delivery and cell plate formation. The phragmoplast expands centrifugally and guides the growing cell plate to the CDZ, where it eventually fuses with the maternal wall to generate a new crosswall. Distinct sets of microtubule-associated proteins (MAPs) regulate PPB and spindle formation, CDZ set up, phragmoplast dynamics and the connection between microtubules and membrane compartments. Several genetic, cell biological, and physiological studies have provided insights into the role of individual MAPs and membrane-associated proteins in microtubule organization and membrane dynamics during cell plate formation. How different MAPs interact with each other, and how microtubule arrays are coordinated with membranes at the CDZ, cell plate and during vesicle delivery, however, remains elusive. Our work in the model plant Arabidopsis thaliana identified plant-specific IQ67 Domain (IQD) proteins as novel MAPs with proposed roles as cellular scaffolds for macromolecular complex assembly. We showed that members of the IQD family determine division plane positioning by controlling PPB formation and recruitment of CDZ-resident proteins. The respective IQD proteins localize to membranes and microtubules at the CDZ and cell plate, and at the PPB and phragmoplast, respectively, and interact with several CDZ-, phragmoplast- and cell plate-localized proteins via distinct motifs and domains adjacent to its eponymous IQ67 domain. The IQ67 domain harbors multiple functional Calmodulin (CaM) binding motifs pointing to a role of IQDs in calcium signaling. How or if IQDs contribute to CaM-mediated calcium signaling at the microtubule cytoskeleton, however, is still unknown.This proposal aims at elucidating the (CaM-regulated) dynamics and composition of IQD-assembled complexes at the phragmoplast and cell plate by combining cell biology, genetics and protein biochemistry. The combined approach will deepen our understanding of the regulation of microtubule organization and its connection with membranes. Thereby, our strategy offers a mechanistic framework to unravel principles governing signal integration at macromolecular protein assemblies during cell division and beyond.

• DFG Programme: Research Grants