A new identity for MLK3 as an NIMA-related, cell cycle-regulated kinase that is localized near centrosomes and influences microtubule organization.
Although conserved counterparts for most proteins involved in the G(2)/M transition of the cell cycle have been found in all eukaryotes, a notable exception is the essential but functionally enigmatic fungal kinase NIMA. While a number of vertebrate kinases have been identified with catalytic domain homology to NIMA, none of these resemble NIMA within its extensive noncatalytic region, a region critical for NIMA function in Aspergillus nidulans. We used a bioinformatics approach to search for proteins with homology to the noncatalytic region of NIMA and identified mixed lineage kinase 3 (MLK3). MLK3 has been proposed to serve as a component in MAP kinase cascades, particularly those resulting in the activation of the c-Jun N-terminal kinase (JNK). Here we describe the first in-depth study of endogenous MLK3 and report that, like NIMA, MLK3 phosphorylation and activity are enhanced during G(2)/M, whereas JNK remains inactive. Coincident with the G(2)/M transition, a period marked by dramatic reorganization of the cytoplasmic microtubule network, endogenous MLK3 transiently disperses away from the centrosome and centrosomal-proximal sites where it is localized during interphase. Furthermore, when overexpressed, MLK3, like NIMA, localizes to the centrosomal region, induces profound disruption of cytoplasmic microtubules and a nuclear distortion phenotype that differs from mitotic chromosome condensation. Cellular depletion of MLK3 protein using siRNA technology results in an increased sensitivity to the microtubule-stabilizing agent taxol. Our studies suggest a new role for MLK3, separable from its function in the JNK pathway, that may contribute to promoting microtubule instability, a hallmark of M phase entry.
Swenson
Katherine I
KI
Department of Pharmacology and Cancer Biology, Duke University Medical Center, Durham, North Carolina 27710, USA.
Winkler
Katharine E
KE
Means
Anthony R
AR
D000595:Amino Acid Sequence; D002453:Cell Cycle / Q000502:physiology*; D018797:Cell Cycle Proteins; D018385:Centrosome / Q000378:metabolism*; D006801:Humans; D020930:MAP Kinase Kinase Kinases / Q000378:metabolism*; D008870:Microtubules / Q000378:metabolism*; D000072380:NIMA-Related Kinase 1; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism; D017385:Sequence Homology; D000097767:Mitogen-Activated Protein Kinase Kinase Kinase 11
D018797:Cell Cycle Proteins; D020930:MAP Kinase Kinase Kinases; D000072380:NIMA-Related Kinase 1; D017346:Protein Serine-Threonine Kinases; D000097767:Mitogen-Activated Protein Kinase Kinase Kinase 11; C000606914:NEK1 protein, human