Oncoprotein 18 (Op18, also termed p19, p18, prosolin or stathmin) is a cytosolic protein of previously unknown function. Phosphorylation of Op18 is cell cycle regulated by cyclin‐dependent kinases (CDKs), and expression of a ‘CDK target site‐deficient mutant’ results in a phenotype indicative of a role for Op18 during mitosis. This phenotype is compatible with the idea that Op18 is a phosphorylation‐responsive regulator of microtubule (MT) dynamics. Therefore, in this study, we analyzed MTs in cells induced to express either wild‐type or mutated Op18. The results showed that wild‐type Op18 and a CDK target site mutant both efficiently elicited rapid depolymerization of MTs. This result contrasts with clear‐cut differences in their cell cycle phenotypes. Morphological analysis of MTs explained this apparent discrepancy: while interphase MTs were depolymerized in cells expressing either Op18 derivative, apparently normal mitotic spindles were formed only in cells overexpressing wild‐type Op18. This result correlates with our finding that only mutated Op18 causes a block during mitosis. Hence, we conclude that Op18 decreases MT stability and that this activity of Op18 is subject to cell cycle regulation by CDKs.

Oncoprotein 18 (Op18; also termed p19, 19K, metablastin, stathmin, and prosolin) is a conserved protein that regulates microtubule (MT) dynamics. Op18 is multisite phosphorylated on four Ser residues during mitosis; two of these Ser residues, Ser-25 and Ser-38, are targets for cyclin-dependent protein kinases (CDKs), and the other two Ser residues, Ser-16 and Ser-63, are targets for an unidentified protein kinase. Mutations of the two CDK sites have recently been shown to result in a mitotic block caused by destabilization of MTs. To understand the role of Op18 in regulation of MT dynamics during mitosis, in this study we dissected the functions of all four phosphorylation sites of Op18 by combining genetic, morphological, and biochemical analyses. The data show that all four phosphorylation sites are involved in switching off Op18 activity during mitosis, an event that appears to be essential for formation of the spindle during metaphase. However, the mechanisms by which specific sites down-regulate Op18 activity differ. Hence, dual phosphorylation on the CDK sites Ser-25 and Ser-38 appears to be required for phosphorylation of Ser-16 and Ser-63; however, by themselves, the CDK sites are of only minor importance in direct regulation of Op18 activity. Subsequent phosphorylation of either Ser-16, Ser-63, or both efficiently switches off Op18 activity.Microtubules (MTs) are polymeric components of the cytoskeleton found in all eukaryotes and are composed of heterodimers of ␣-and ␤-tubulin. In nondividing cells and during the interphase of the cell cycle, MTs are important for organizing the cytoplasm, for organelle transport, and for intracellular movement of cell surface receptors (for a review, see reference 7). During mitosis, large arrays of MTs, termed the mitotic spindle, segregate the condensed chromosomes (for a review, see reference 17).Tubulin exists in a dynamic equilibrium between free tubulin dimers and MTs. The dynamic instability model describes the dynamic behavior of MTs in terms of the following four parameters: the rates of growth and shrinkage of tubulin polymers and the frequencies of catastrophes (transitions from growth to shrinkage) and rescues (transitions from shrinkage to growth) (30). The dynamic instability of MTs is in part regulated by MT-associated proteins (MAPs), most of which are phosphoproteins (for a review, see reference 15). It is thought that signal transduction cascades that regulate cell proliferation and differentiation control MT dynamics by phosphorylation of MAPs. The consensus in the field is that MAPs stabilize MTs by direct binding and that phosphorylation of MAPs decreases the binding affinity toward MTs and thereby weakens their stabilizing effect (10, 36).

Oncoprotein 18 (Op18; also termed p19, 19K, p18, prosolin, and stathmin) is a regulator of microtubule (MT) dynamics and is phosphorylated by multiple kinase systems on four Ser residues. In addition to cell cycle-regulated phosphorylation, external signals induce phosphorylation of Op18 on Ser-25 by the mitogen-activated protein kinase and on Ser-16 by the Ca2+/calmodulin-dependent kinase IV/Gr (CaMK IV/Gr). Here we show that induced expression of a constitutively active mutant of CaMK IV/Gr results in phosphorylation of Op18 on Ser-16. In parallel, we also observed partial degradation of Op18 and a rapid increase of total cellular MTs. These results suggest a link between CaMK IV/Gr, Op18, and MT dynamics. To explore such a putative link, we optimized a genetic system that allowed conditional coexpression of a series of CaMK IV/Gr and Op18 derivatives. The result shows that CaMK IV/Gr can suppress the MT-regulating activity of Op18 by phosphorylation on Ser-16. In line with these results, by employing a chemical cross-linking protocol, it was shown that phosphorylation of Ser-16 is involved in weakening of the interactions between Op18 and tubulin. Taken together, these data suggest that the mechanism of CaMK IV/Gr-mediated suppression of Op18 activity involves both partial degradation of Op18 and direct modulation of the MT-destabilizing activity of this protein. These results show that Op18 phosphorylation by CaMK IV/Gr may couple alterations of MT dynamics in response to external signals that involve Ca2+.

Oncoprotein 18 (Op18, also termed p19, 19K, metablastin, stathmin, and prosolin) is a recently identified regulator of microtubule (MT) dynamics. Op18 is a target for both cell cycle and cell surface receptor-coupled kinase systems, and phosphorylation of Op18 on specific combinations of sites has been shown to switch off its MT-destabilizing activity. Here we show that induced expression of the catalytic subunit of cAMP-dependent protein kinase (PKA) results in a dramatic increase in cellular MT polymer content concomitant with phosphorylation and partial degradation of Op18. That PKA may regulate the MT system by downregulation of Op18 activity was evaluated by a genetic system allowing conditional co-expression of PKA and a series of kinase target site–deficient mutants of Op18. The results show that phosphorylation of Op18 on two specific sites, Ser-16 and Ser-63, is necessary and sufficient for PKA to switch off Op18 activity in intact cells. The regulatory importance of dual phosphorylation on Ser-16 and Ser-63 of Op18 was reproduced by in vitro assays. These results suggest a simple model where PKA phosphorylation downregulates the MT-destabilizing activity of Op18, which in turn promotes increased tubulin polymerization. Hence, the present study shows that Op18 has the potential to regulate the MT system in response to external signals such as cAMP-linked agonists.

Oncoprotein 18/stathmin (Op18) is a recently identified phosphorylation-responsive regulator of the microtubule (MT) system. It was originally proposed that Op18 specifically regulates dynamic properties of MTs by associating with tubulin, but it has subsequently been proposed that Op18 acts simply by sequestering of tubulin heterodimers. We have dissected the mechanistic action of Op18 by generation of two distinct classes of mutants. One class has interruptions of the heptad repeats of a potential coiled-coil region of Op18, and the other involves substitution at all four phosphorylation sites with negatively charged Glu residues. Both types of mutation result in Op18 proteins with a limited decrease in tubulin complex formation. However, the MT-destabilizing activities of the coiled-coil mutants are more severely reduced in transfected leukemia cells than those of the Glu-substituted Op18 derivative, providing evidence for tubulin-directed regulatory activities distinct from tubulin complex formation. Analysis of Op18-mediated regulation of tubulin GTPase activity and taxol-promoted tubulin polymerization showed that while wild-type and Glu-substituted Op18 derivatives are active, the coiled-coil mutants are essentially inactive. This suggests that Op18-tubulin contact involves structural motifs that deliver a signal of regulatory importance to the MT system. Microtubules (MTs) participate in a variety of cellular processes, including chromosome segregation during mitosis, cell motility, and intracellular vesicle transport. MTs are known to be ever-changing dynamic structures that switch abruptly between elongation and shortening. The switch from growth to shortening is called catastrophe, and the switch between shortening and growth is called rescue (for a review, see reference 8). Classically, regulation of MT dynamics has been ascribed to a class of nonmotor proteins collectively termed MT-associated proteins (MAPs). More recently, a family of MT motors has been shown to regulate MT dynamics both in vivo and in vitro (for a review, see reference 15). Besides these two classes of MT regulators, it has recently been shown that a cytosolic protein termed oncoprotein 18/stathmin (Op18) regulates MT dynamics both in vitro (2) and in intact cells (13,21).Several lines of evidence suggest that Op18 is an important phosphorylation-responsive regulator of the MT system in intact cells (for a review, see reference 18). Phosphorylation by either cell surface receptor or cell cycle-regulated kinase systems on four distinct Ser residues decreases the MT-directed activity of Op18 both in vitro and in intact cells (10,17,23). The kinase systems involved have been identified as members of the mitogen-activated protein kinase (MAPK), CaM kinase IV/Gr (CaMK IV/Gr), cyclic AMP-dependent kinase (PKA), and cyclin-dependent kinase (CDK) families (for a review, see reference 7).Op18 has been identified as a factor that both forms complexes with tubulin heterodimers and destabilizes MTs by promoting catastrophes (2). However, the...