Membrane dynamics in cytokinesis

Figure 5. Evolution of cytokinetic SNARE complexes 

(A) Phylogenetic tree of SYP1 Qa-SNAREs in plant evolution

(B) With the advent of the angiosperms, cytokinesis-specific KNOLLE SNARE complexes were added to the ancient SYP132 SNARE complexes serving secretion and cytokinesis (Park et al., 2018).

           The KNOLLE-containing SNARE complexes already form at the ER membrane and are delivered as cis-SNARE complexes to the plane of cell division by membrane trafficking via Golgi stacks and TGN (Fig. 6; Karnahl & Park et al., 2017).

Figure 6. Trafficking of cis-SNARE complexes during cytokinesis (model).

Two different types of cytokinetic cis-SNARE complexes are assembled on the ER, recruited into COPII vesicles and passed on to the Golgi stack/TGN. At the TGN, the cis-SNARE complexes are incorporated into AP1/CCV vesicles for delivery to the division plane. Following their disassembly by NSF ATPase, monomeric Qa-SNARE KNOLLE is assisted by SM protein KEULE in the formation of trans-SNARE complexes mediating fusion of adjacent vesicles during cell-plate formation and expansion (Karnahl & Park et al., 2017).

           Cell-plate formation involves newly-synthesised proteins including KNOLLE as well as proteins endocytosed from the plasma membrane which are recycled back to the plasma membrane in interphase. During cytokinesis, however, recycling of endocytosed PM proteins does not occur. These proteins are instead re-directed along a late-secretory pathway dependent on ARF-GEFs BIG1-4 to the plane of cell division (Richter et al., 2014).

            Before vesicle fusion during cell-plate formation, the cis-SNARE complexes are disassembled by the action of AAA ATPase NSF and its co-factor aSNAP2 (Fig. 7; Park et al., 2023). Monomeric KNOLLE is then prevented from adopting an inactive "closed" state by interaction with the regulatory Sec1p/Munc18 (SM) protein KEULE (KEU), which promotes trans-SNARE complex formation between neighbouring membrane vesicles (Park et al., 2012).

Figure 7. A mechanistic model of membrane fusion in Arabidopsis cell-plate formation involving disassembly of  cis-SNARE complexes by NSF ATPase and formation of trans-SNARE complexes promoted by SM protein KEULE. 

TGN-derived vesicles are delivered to the forming cell plate along the microtubules of the phragmoplast (not drawn). Hexameric NSF ATPase (bright green) and four copies of αSNAP2 adaptor (bright blue) form a 20S particle with the cis-SNARE complex (Qa-SNARE KNOLLE, red; Qbc-SNARE SNAP33, sand brown; R-SNARE VAMP721 or 722, dark blue) on each vesicle. ATP hydrolysis disassembles the cis-SNARE complex into monomeric SNAREs (broken lines, indicating a very transient state). The SM protein KEULE (pale green) interacts with monomeric fusion-competent KNOLLE. This interaction accelerates trans-SNARE complex formation between KNOLLE and its SNARE partners on adjacent vesicles, which mediates their fusion. (Park et al., 2023).

            Our current research addresses membrane dynamics in cytokinesis. To this end, we analyse how KN is targeted to its site of action and turned over, how KN interacts with other proteins, what cargo proteins are specifically delivered to the forming cell plate and how they act during cytokinesis.