Identification of Myosin XI Receptors in Arabidopsis Defines a Distinct Class of Transport Vesicles  

Abstract

To characterize the mechanism through which myosin XI-K attaches to its principal endomembrane cargo, a yeast two-hybrid library of Arabidopsis thaliana cDNAs was screened using the myosin cargo binding domain as bait. This screen identified two previously uncharacterized transmembrane proteins (hereinafter myosin binding proteins or MyoB1/2) that share a myosin binding, conserved domain of unknown function 593 (DUF593). Additional screens revealed that MyoB1/2 also bind myosin XI-1, whereas myosin XI-I interacts with the distantly related MyoB7. The in vivo interactions of MyoB1/2 with myosin XI-K were confirmed by immunoprecipitation and colocalization analyses. In epidermal cells, the yellow fluorescent protein–tagged MyoB1/2 localize to vesicles that traffic in a myosin XI–dependent manner. Similar to myosin XI-K, MyoB1/2 accumulate in the tip-growing domain of elongating root hairs. Gene knockout analysis demonstrated that functional cooperation between myosin XI-K and MyoB proteins is required for proper plant development. Unexpectedly, the MyoB1-containing vesicles did not correspond to brefeldin A–sensitive Golgi and post-Golgi or prevacuolar compartments and did not colocalize with known exocytic or endosomal compartments. Phylogenomic analysis suggests that DUF593 emerged in primitive land plants and founded a multigene family that is conserved in all flowering plants. Collectively, these findings indicate that MyoB are membrane-anchored myosin receptors that define a distinct, plant-specific transport vesicle compartment.

Myosin-driven transport network in plants

“…Because the frequency with which the cDNA clones are identified in Y2H screens depends on their library representation, these screens are prone to bias, as indicated by the fact that the most abundant hits were observed with MyoB1 and MyoB2, which are expressed to the highest levels throughout Arabidopsis tissues. To overcome this bias, we conducted directed, pairwise Y2H assays that involved the same five myosins and 11 MyoBs representing each of the six MyoB subfamilies (14). Seven of these MyoBs that produced no hits in the previous library screens were included to determine whether myosin binding is indeed a common property of MyoB family proteins.…”

“…The latter hypothesis was further boosted by the discovery of the bona fide myosin receptors, the MyoB family proteins, which colocalized with myosin XI-K to the distinct vesicular compartment described above (14). The MyoB protein family is characterized by the universally conserved, myosin-binding, coiled-coil domain (previously known as DUF593); most of the MyoBs also contain a predicted transmembrane helix (TM) that is sufficient for MyoB anchoring to endomembrane vesicles in a myosindependent manner (14).…”

“…In addition, it has been shown that the MyoB compartment belonged neither to the ER-to-Golgi-toplasma membrane secretory pathway nor to several other known exo-or endocytic compartments, with the implication that the myosin-MyoB complex defines a previously undescribed, specialized transport compartment, a potential driver of cytoplasmic streaming (14). The MyoB family is conserved in all land plants and some green algae, but not in other eukaryotes (14). Despite the advanced status of the yeast model, analogous bona fide, membrane-anchored, vesicular myosin receptors remain to be identified (15), although a variety of fungal and animal myosin V adaptors and regulators have been characterized in distinct cell types (16).…”

“…The MyoB protein family is characterized by the universally conserved, myosin-binding, coiled-coil domain (previously known as DUF593); most of the MyoBs also contain a predicted transmembrane helix (TM) that is sufficient for MyoB anchoring to endomembrane vesicles in a myosindependent manner (14). In addition, it has been shown that the MyoB compartment belonged neither to the ER-to-Golgi-toplasma membrane secretory pathway nor to several other known exo-or endocytic compartments, with the implication that the myosin-MyoB complex defines a previously undescribed, specialized transport compartment, a potential driver of cytoplasmic streaming (14). The MyoB family is conserved in all land plants and some green algae, but not in other eukaryotes (14).…”

Myosins XI modulate host cellular responses and penetration resistance to fungal pathogens

“…Myosins are molecular motors responsible for AF-based motility (20). Recently, plant class XI myosins were implicated in the organization of actin cytoskeleton, organelle and vesicle transport, cell expansion, and plant growth (21)(22)(23)(24)(25)(26)(27). Although none of the individual myosin gene knockouts produces plant growth defects (22), progressive elimination of two to four highly expressed myosins results in concomitant reduction in cell and plant size (23,24).…”