A MAP Kinase Is Activated Late in Plant Mitosis and Becomes Localized to the Plane of Cell Division

Abstract

In eukaryotes, mitogen-activated protein kinases (MAPKs) are part of signaling modules that transmit diverse stimuli, such as mitogens, developmental cues, or various stresses. Here, we report a novel alfalfa MAPK, Medicago MAP kinase 3 (MMK3). Using an MMK3-specific antibody, we detected the MMK3 protein and its associated activity only in dividing cells. The MMK3 protein could be found during all stages of the cell cycle, but its protein kinase activity was transient in mitosis and correlated with the timing of phragmoplast formation. Depolymerization of microtubules by short treatments with the drug amiprophosmethyl during anaphase and telophase abolished MMK3 activity, indicating that intact microtubules are required for MMK3 activation. During anaphase, MMK3 was found to be concentrated in between the segregating chromosomes; later, it localized at the midplane of cell division in the phragmoplast. As the phragmoplast microtubules were redistributed from the center to the periphery during telophase, MMK3 still localized to the whole plane of division; thus, phragmoplast microtubules are not required to keep MMK3 at this location. Together, these data strongly support a role for MMK3 in the regulation of plant cytokinesis. INTRODUCTIONPlant shape is elaborated by developmental signals regulating the timing and orientation of cell division and cell enlargement. Many components of cell cycle control are conserved evolutionarily among yeasts, animals, and plants, including the cyclin-dependent protein kinases, which are key regulators of cell cycle progression (Hirt and Heberle-Bors, 1994). The spatial control of cell division, on the other hand, is largely dependent on plant-specific cytoskeletal structures, such as the prepophase band, which marks the division plane before mitosis, and the phragmoplast, which is required to lay down the new cell wall during cytokinesis (Wick, 1991).Little is known about how these cytoskeletal arrays are constructed and function during cell division, but evidence is accumulating that protein phosphorylation plays an important role (Katsuta and Shiboka, 1992;Wolniak and Larsen, 1995).The mitogen-activated protein kinases (MAPKs) are utilized in eukaryotes to transmit diverse stimuli, such as mitogens, developmental cues, or various stresses (Robinson and Cobb, 1997). They are integrated into a signaling module composed of three linked protein kinases: the MAPK, the MAPK-activating kinase (MEK), and the MEK-activating kinase (MEKK). The functions of MAPKs are best understood in yeast, for which there are six different genes encoding MAPKs. Five of these have been assigned to specific signaling pathways, such as pheromone response, filamentation, response to hypotonic and hypertonic shock, and sporulation (Madhani and Fink, 1998). The genetic dissection of these pathways is not always straightforward, because a MAPK normally assigned to one pathway can overtake the function in another if the relevant MAPK is knocked out by a loss-of-function mutation (Madhani and Fink, 1998). In ...

Plants, MEN and SIN

“…The MAP kinase NtNTF6 and the MAP kinase kinase kinase (MAP3K) NtNPK1 from tobacco, activated at the end of mitosis, localized to the equatorial zone of the phragmoplast and to the cell plate [6,26]. The overexpression of a kinase-defective mutant form of NtNPK1 produces multinucleate cells with incomplete cell plates, a result consistent with the view that NtNPK1 is involved in phragmoplast expansion during cytokinesis [26].…”

Alfalfa Mob1-like proteins are involved in cell proliferation and are localized in the cell division plane during cytokinesis

“…Proteins encoded by some of these cloned genes have a cell distribution pattern similar to that of alfalfa Mob1-like proteins. This is the case of Tangled1 protein from corn [44], which is involved in orienting cytoskeletal structures during cell division, and MMK3 kinase from alfalfa [45], whose function in plant cytokinesis is still to be determined. Phragmoplastin, a dynamin-like protein from soybean, is associated with cell plate formation [46,47] but, unlike MMK3 and Mob1-like proteins, it first appears in the plate centre and as the plate grows outwards it redistributes to the growing septum margins.…”

ENDOSPERM DEVELOPMENT: Cellularization and Cell Fate Specification

“…Recent work also suggests a role for MAP kinases in cytokinesis. The two closely related MAP kinases NTF6 (32) and MMK3 (14) from tobacco and alfalfa, respectively, are specifically activated during M-phase. Here they reach peak activity during cytokinesis, when they are present in the cell plate, suggesting that they regulate the activity of protein(s) involved in cell plate formation.…”