Myosin-driven transport network in plants

Abstract

We investigate the myosin XI-driven transport network in Arabidopsis using protein-protein interaction, subcellular localization, gene knockout, and bioinformatics analyses. The two major groups of nodes in this network are myosins XI and their membrane-anchored receptors (MyoB) that, together, drive endomembrane trafficking and cytoplasmic streaming in the plant cells. The network shows high node connectivity and is dominated by generalists, with a smaller fraction of more specialized myosins and receptors. We show that interaction with myosins and association with motile vesicles are common properties of the MyoB family receptors. We identify previously uncharacterized myosin-binding proteins, putative myosin adaptors that belong to two unrelated families, with four members each (MadA and MadB). Surprisingly, MadA1 localizes to the nucleus and is rapidly transported to the cytoplasm, suggesting the existence of myosin XI-driven nucleocytoplasmic trafficking. In contrast, MadA2 and MadA3, as well as MadB1, partition between the cytosolic pools of motile endomembrane vesicles that colocalize with myosin XI-K and diffuse material that does not. Gene knockout analysis shows that MadB1-4 contribute to polarized root hair growth, phenocopying myosins, whereas MadA1-4 are redundant for this process. Phylogenetic analysis reveals congruent evolutionary histories of the myosin XI, MyoB, MadA, and MadB families. All these gene families emerged in green algae and show concurrent expansions via serial duplication in flowering plants. Thus, the myosin XI transport network increased in complexity and robustness concomitantly with the land colonization by flowering plants and, by inference, could have been a major contributor to this process.myosins | receptors | adaptors | cytoplasmic streaming | nuclear transport F or half of a century, it had been assumed that rapid organelle trafficking and cytoplasmic streaming in plant cells rely on myosin motors (1) and, in particular, class XI myosins that are homologous to fungal and animal class V myosins (2). Over the past decade, a massive amount of information on specific functions of myosins and mechanisms of myosin XI-dependent processes in plants has been obtained (3-5). The first genetic evidence of myosin function in cell expansion involved the demonstration of a dramatic reduction of the polarized root hair growth upon inactivation of the myosin XI-K and XI-2 genes (6, 7). Subsequently, it has been shown that myosindependent trafficking is required for the growth of multiple cell types, as well as normal plant growth and development (4,(8)(9)(10). Among the 13 paralogous myosins XI encoded in the Arabidopsis thaliana genome, the highly expressed myosins XI-K, XI-1, and XI-2 have been shown to provide the most pronounced, functionally redundant contributions to each of these processes. In contrast, another highly expressed paralog, myosin XI-I, plays only limited roles in cell and plant growth but has a specialized function in nuclear repositioning through binding nuclear e...

Update on Myosin Motors: Molecular Mechanisms and Physiological Functions

“…In this case, all tested motors spanning a range of myosin XI subtypes were found to associate with DCP1 under a variety of experimental conditions, including bimolecular fluorescence complementation in plant cells (Steffens et al, 2014). The only case of specific binding between a motor and a cargo protein described to date is the interaction between Myo11G and WIT1/2 (Tamura et al, 2013;Kurth et al, 2017), suggesting that this early branching myosin XI has evolved a unique function in nuclear shape and movement. This interpretation also is consistent with Myo11G's very slow ATP hydrolysis cycle (Haraguchi et al, 2016).…”

“…Curiously, this does not seem to be the case. For example, a recent broad analysis of pairwise interactions by yeast twohybrid assay revealed remarkably little specificity in binding among five Myo11 and 10 MyoB isoforms (Kurth et al, 2017). A similar analysis for MadA proteins showed some more specificity, but at least one family member, MadA1, interacted nonselectively with all tested myosins (Kurth et al, 2017).…”

“…The localization of fluorescent protein-tagged MyoB proteins revealed their presence on numerous small, punctate structures that moved rapidly through cells in a pattern that was very similar to Myo11E-labeled spots (Peremyslov et al, 2013;Kurth et al, 2017). A tobacco homolog of the MyoB family, RISAP, was found associated with the trans-Golgi network in pollen tubes (Stephan et al, 2014), but none of the tested Arabidopsis MyoB proteins colocalized with a variety of established organelle markers in Arabidopsis (Peremyslov et al, 2013;Kurth et al, 2017). It remains to be seen whether these MyoB compartments define a novel set of endomembrane vesicles that were previously unknown or whether they represent subdomains of known organelles.…”

“…MyoB proteins form large families in both dicots (Arabidopsis) and monocots (Oryza sativa), and subfamilies are predicted to carry various additional domains in addition to DUF593, such as transmembrane domains, coiled-coil regions, or metal-binding regions (Peremyslov et al, 2013). The localization of fluorescent protein-tagged MyoB proteins revealed their presence on numerous small, punctate structures that moved rapidly through cells in a pattern that was very similar to Myo11E-labeled spots (Peremyslov et al, 2013;Kurth et al, 2017). A tobacco homolog of the MyoB family, RISAP, was found associated with the trans-Golgi network in pollen tubes (Stephan et al, 2014), but none of the tested Arabidopsis MyoB proteins colocalized with a variety of established organelle markers in Arabidopsis (Peremyslov et al, 2013;Kurth et al, 2017).…”

“…They include two groups of proteins, MadA and MadB, that lack a transmembrane domain and, therefore, are assumed to function as adaptors between motors and other proteins on organelle surfaces (Kurth et al, 2017). Furthermore, an interaction between myosin XI and DECAPPING PROTEIN1 (DCP1; Steffens et al, 2014), a central component of processing bodies that are centers of RNA processing in cells (Bailey-Serres et al, 2009), was detected.…”