For comprehensive understanding of actin myosin XI
For comprehensive understanding of actin-myosin XI as a control network, it will be necessary to determine the functions of all the myosin XI and Chloroquine isoforms. However, it is difficult to reveal the function of individual myosin XI isoforms because most myosin XI single knockouts exhibited no significant phenotype in Arabidopsis. Our previous study established a technique to produce chimeric myosin XI-2 with an altered speed by replacing the original motor domain of Arabidopsis myosin XI-2 with high- or low-speed motors. The transgenic Arabidopsis expressing the chimeric myosin XI-2 indicated remarkable phenotypes and an apparent relationship between cytoplasmic streaming and plant growth . This chimeric myosin XI system provides a powerful tool to elucidate the specific functions of individual myosin XI isoforms, because the chimeric myosin XIs that used the same tail domain sustained the binding ability to specific cargo of native myosin XIs in Arabidopsis. As more information about the enzymatic and motile activities, genetic analysis in gene knockouts and the chimeric myosin XIs becomes available, future research will elucidate the mechanism of actin–myosin XI cytoskeleton for intracellular transport in various tissues of higher plants in details.
Conflicts of interest
Main Text Actin filaments drive diverse cellular processes by generating force. Actin is well established to generate force by associating with motor proteins, or by creating pushing forces through filament polymerization. Prominent examples for these two mechanisms can be found during cell migration: actin polymerization creates pushing forces that drive the protrusion of the leading edge of the cell, whereas actomyosin contraction enables movement of the cell rear . In addition to these two mechanisms, several recent studies have suggested that actin can also create force by disassembly 2, 3, 4, 5. However, the in vivo evidence for force generation by actin disassembly has still been limited. A recent study by Bun et al. now provides compelling evidence that disassembly drives the contraction of an actin network that is required to transport chromosomes to the first meiotic spindle in starfish oocytes (Figure 1A). In many species, oocytes have exceptionally large nuclei, reaching diameters of around 80 μm in starfish and 450 μm in frogs. This creates a problem for the oocyte as the length of microtubules that need to capture the chromosomes upon nuclear envelope breakdown is typically limited to 30 μm — too short to reach all of the chromosomes within the nucleus . Thus, cells with large nuclei need to employ additional mechanisms to ensure that all chromosomes can be reached by microtubules. Starfish oocytes use a ‘fishing net’ made of actin filaments to collect chromosomes from the large nucleus and bring them into the proximity of the forming spindle . At nuclear envelope breakdown, the actin network forms throughout the nucleus . Contraction of the network then delivers chromosomes to the spindle. Disruption of the actin network causes chromosomes to be lost in the cytoplasm, precluding the development of a healthy embryo upon fertilization. But how is the actin network that is transporting the chromosomes contracting? By using elegant pulse-chase labelling experiments, Bun et al. observed that, while the initial network contracted, new actin filaments were polymerized at the surface of this contracting mesh. They hence investigated whether these new filaments might generate pushing forces that could compact the network and thereby transport the chromosomes. They reasoned that, if the network was continuously compacted from the outside, then artificially induced gaps in the network should be rapidly closed. However, local laser ablation of the network resulted in rapid retraction of filaments from the ablation site and an enlargement of the gap. This suggested that network contraction is not driven by pushing forces from the outside, but that intrinsic contractility within the network generates the forces that move the chromosomes.