The microtubule-associated kinase-like protein RUNKEL functions in somatic and syncytial cytokinesis.

The microtubule (MT)-associated putative kinase RUNKEL (RUK) is an important component of the phragmoplast machinery involved in cell plate formation in Arabidopsis somatic cytokinesis. Since loss-of-function ruk mutants display seedling lethality, it was previously not known whether RUK functions in mature sporophytes or during gametophyte development. In this study we utilized RUK proteins that lack the N-terminal kinase domain to further examine biological processes related to RUK function. Truncated RUK proteins when expressed in wild-type Arabidopsis plants cause cellularization defects not only in seedlings and adult tissues but also during male meiocyte development, resulting in abnormal pollen and reduced fertility. Ultrastructural analysis of male tetrads revealed irregular and incomplete or absent intersporal cell walls, caused by disorganized radial MT arrays. Moreover, in ruk mutants endosperm cellularization defects were also caused by disorganized radial MT arrays. Intriguingly, in seedlings expressing truncated RUK proteins, the kinesin HINKEL, which is required for the activation of a mitogen-activated protein kinase signaling pathway regulating phragmoplast expansion, was mislocalized. Together, these observations support a common role for RUK in both phragmoplast-based cytokinesis in somatic cells and syncytial cytokinesis in reproductive cells.

Microtubule-associated kinase-like protein RUNKEL needed [corrected] for cell plate expansion in Arabidopsis cytokinesis.

Cytokinesis partitions the cytoplasm of dividing eukaryotic cells. In higher plants, a dynamic microtubule array--phragmoplast--mediates the formation of the partitioning membrane--cell plate--in a centrifugal fashion. This phragmoplast dynamic involves microtubule-associated proteins. Mutations in a novel Arabidopsis gene RUNKEL (RUK) result in cytokinesis defects caused by abnormal phragmoplast organization and arrested cell plate expansion. RUK encodes an essential cell-cycle-regulated 152 kDa protein with a putative serine/threonine kinase domain and a large microtubule-binding domain, both of which are largely conserved in uncharacterized proteins from protozoa, plants, and animals. RUK directly bound to microtubules in vitro and colocalized with mitotic preprophase band, spindle, and phragmoplast in vivo. An engineered RUK fusion protein that was degraded before telophase did not rescue the ruk mutant phenotype, demonstrating RUK action during cytokinesis. Both microtubule-binding domain and putative kinase domain were essential for RUK function. Surprisingly, RUK did not show kinase activity in vitro, and transgenically expressed "kinase-dead" RUK rescued the seedling lethality of ruk mutants. Our results suggest that RUK plays a regulatory, rather than catalytic, role in phragmoplast microtubule organization during cell plate expansion in cytokinesis.