ReviewThe oncoprotein 18/stathmin family of microtubule destabilizers
Introduction
Proteins that destabilize microtubules have been identified recently. These proteins probably increase microtubule turnover in cells, contributing to rapid reorganization of the microtubule cytoskeleton. One of these microtubule destabilizing protein families, the oncoprotein 18/stathmin (Op18/stathmin) family, is the subject of this review.
Op18/stathmin was initially identified as a protein phosphorylated in response to several extracellular signals [1] and highly over-expressed in leukemias [2]. It is also over-expressed in highly proliferative breast cancers [3] and malignant ovarian cancers [4]. Decreasing the level of Op18/stathmin reverses many of the phenotypes associated with transformation; cancer cells able to grow independently of substrate anchorage or serum were no longer able to do so after Op18/stathmin reduction [5]. Decreasing Op18/stathmin expression also blocks nerve growth factor-stimulated neuronal differentiation of PC12 cells [6]. These observations are consistent with the two names given to this protein; ‘oncoprotein’ obviously refers to its association with cancer and ‘stathmin’ is derived from stathmos, the Greek word for ‘relay’, reflecting this protein's role as an intermediate in signal transduction. Stathmos can also be translated as ‘terminal’ or ‘stop’, and this translation loosely fits the microtubule destabilizing function (‘terminating’ or ‘stopping’ microtubule growth) identified by Belmont and Mitchison [7].
Op18/stathmin is a soluble, cytoplasmic protein with a fraction localized to punctate spots in the cytoplasm and to spindle poles during mitosis 8., 9.. The 149 amino acid protein is highly conserved among vertebrates. For example, human and Xenopus Op18/stathmin are 79% identical [10]. The intracellular concentration of Op18/stathmin varies considerably among different cell types, ranging from <0.005% to 0.5% of total cell protein, with the highest expression in acute lymphocytic leukemias [11].
In vertebrates, Op18/stathmin is expressed in cells with the potential to proliferate as well as in neurons, but its expression level is reduced in other terminally differentiated cells 11., 12.. Proteins related to Op18/stathmin are also expressed in the nervous system and include SCG10, SCLIP, RB3 and its two splice variants RB3′ and RB3′ 12., 13., 14•.. The neuronal proteins all contain a domain highly homologous to Op18/stathmin and also have additional sequences at their N termini that probably anchor them to membranes [13] (Fig. 1). Each protein is capable of destabilizing microtubules 8., 14•., 15..
Section snippets
How does Op18/stathmin destabilize microtubules?
How Op18/stathmin destabilizes microtubules is currently under debate. Two different mechanisms have been identified in vitro: Op18/stathmin either sequesters tubulin dimers or stimulates microtubule plus end catastrophes 7., 16., 17•., 18.. A protein that sequesters tubulin dimers would reduce the concentration of tubulin available for assembly. Thus, addition of a sequestering protein would have the same effect as lowering the tubulin concentration and would slow the rate of microtubule
Properties of the Op18/stathmin–tubulin complex
To understand how Op18/stathmin can sequester tubulin dimers or stimulate catastrophes, it is critical to know how Op18/stathmin and tubulin interact. A complex between Op18/stathmin and tubulin, termed T2S to reflect the molar ratio of tubulin to Op18/stathmin has been detected by a number of methods including plasmon resonance [16], gel filtration [16], sedimentation velocity [18], coprecipitation 17•., 21•. and electron microscopy ([22••]; L Cassimeris, MGullberg, H Erickson, unpublished
Catastrophe promotion
How Op18/stathmin stimulates catastrophes is not yet understood. Little or no Op18/stathmin co-pellets with microtubules 7., 31., so any binding to microtubules must be weak. It should be noted that SCG10 has been reported to co-purify with microtubules [15], suggesting that at least one family member can recognize and bind tubulin dimers within the microtubule lattice. Catastrophe promotion by Op18/stathmin probably requires GTP hydrolysis at microtubule tips since Op18/stathmin does not
Op18/stathmin destabilizes microtubules in cells and egg extracts
Changing Op18/stathmin levels in living cells or Xenopus egg extracts changes the microtubule polymer level. Increasing the level of Op18/stathmin through over-expression or microinjection of the bacterially expressed protein decreases microtubule polymer in interphase cells 8., 20., 21•., 28•., 34., 35., Xenopus embryos [9] and Xenopus egg extracts 9., 36.. Likewise, SCG10, SCLIP, RB3 and RB3′′ over-expression in HeLa cells also results in microtubule depolymerization 8., 15.. Conversely,
Op18/stathmin is turned off by phosphorylation
Op18/stathmin is phosphorylated in response to a number of signals, including those for cell proliferation and differentiation [41], and progression through the cell cycle 34., 35., 42., 43.. Phosphorylation occurs on four serine residues, Ser16, Ser25, Ser38 and Ser63 (Fig. 1, Table 1). The Xenopus homolog lacks a serine at position 63, and thus contains only three phosphorylation sites 10., 44.. An expanding list of kinases phosphorylate one or more of the serines (summarized in Table 1).
Conclusions
We still have much to learn about how Op18/stathmin and the membrane-associated neuronal family members destabilize microtubules and how these proteins are regulated in cells. The study of Op18/stathmin should provide insights into fundamental aspects of microtubule assembly, such as how microtubule tip structure contributes to the transitions between growth and shortening [33•]. Debates over sequestering versus catastrophe promotion are likely to continue, but we may also need to consider new
Acknowledgements
Thanks to Vincent VanBuren, Martin Gullberg and David Odde for many helpful discussions. Thanks also to Kathryn Cassimeris and Irene Glinos for Greek translations and Kathryn Goettge for editing and improving the English.
References and recommended reading
Papers of particular interest, published within the annual period of review, have been highlighted as:
• of special interest
•• of outstanding interest
References (61)
- et al.
Distinct patterns of cytoplasmic protein phosphorylation related to regulation of synthesis and release of prolactin by GH cells
J. Biol. Chem.
(1983) - et al.
Identification of a polypeptide associated with the malignant phenotype in acute leukemia
J. Biol. Chem.
(1988) - et al.
Identification of a protein that interacts with tubulin dimers and increases the catastrophe rate of microtubules
Cell
(1996) - et al.
Differential effect of two stathmin/Op18 phosphorylation mutants on Xenopus embryo development
J. Biol. Chem.
(2001) - et al.
Stathmin gene family: phylogenetic conservation and development regulation in Xenopus
J. Biol. Chem.
(1993) - et al.
Stathmin family proteins display specific molecular and tubulin binding properties
J. Biol. Chem.
(2001) - et al.
The stathmin/tubulin interaction in vitro
J. Biol. Chem.
(1997) - et al.
The 4 Å X-ray structure of a tubulin: stathmin-like domain complex
Cell
(2000) - et al.
Probing the native structure of stathmin and its interaction domains with tubulin
J. Biol. Chem.
(2000) - et al.
High-resolution model of the microtubule
Cell
(1999)
Stathmin a relay phosphoprotein for multiple signal transduction?
Trends. Biochem. Sci.
G2/M transition requires multisite phosphorylation of oncoprotein 18 by two distinct protein kinase systems
J. Biol. Chem.
Phosphorylation regulates the microtubule-destabilizing activity of stathmin and its interaction with tubulin
FEBS. Lett.
Kin I kinesins are microtubule-destabilizing enzymes
Cell
Tumor necrosis factor-induced microtubule stabilization mediated by hyperphosphorylated oncoprotein 18 promotes cell death
J. Biol. Chem.
Multiple signal transduction pathways induce phosphorylation of serines 16, 25, and 38 of oncoprotein 18 in T lymphocytes
J. Biol. Chem.
Multiple phosphorylation of stathmin. Identification of four sites phosphorylated in intact cells and in vitro by cyclic AMP-dependent protein kinase and p34cdc2
J. Biol. Chem.
Identification of stathmin as a novel substrate for p38 delta
Biochem. Biophys. Res. Commun.
Rac/Cdc42 and p65PAK regulate the microtubule-destabilizing protein stathmin through phosphorylation at serine 16
J. Biol. Chem.
Overexpressioin of stathmin in breast carcinomas points out to highly proliferative tumours
Br. J. Cancer.
The phosphoprotein Op18/stathmin is differentially expressed in ovarian cancer
Cancer. Invest.
Antisense RNA inhibition of phosphoprotein p18 expression abrogates the transformed phenotype of leukemia cells
Cancer. Res.
The phosphoprotein stathmin is essential for nerve growth factor-stimulated differentiation
J. Cell. Biol.
The stathmin phosphoprotein family: intracellular localization and effects on the microtubule network
J. Cell. Sci.
Quantitative analysis of the expression and regulation of an activation-regulated phosphoprotein (oncoprotein 18) in normal and neoplastic cells
Leukemia
Stathmin expression is a feature of proliferating cells of most, if not all, cell lineages
Lab. Invest.
The stathmin family. Molecular and biological characterization of novel mammalian proteins expressed in the nervous system
Eur. J. Biochem.
Regulation of microtubule dynamics by the neuronal growth-associated protein SCG10
Proc. Natl. Acad. Sci. USA
Dissociation of the tubulin-sequestering and microtubule catastrophe-promoting activities of oncoprotein 18/stathmin
Mol. Biol. Cell.
Stathmin: a tubulin-sequestering protein which forms a ternary T2S complex with two tubulin molecules
Biochemistry
Cited by (369)
Novel roles of karyopherin subunit alpha 2 in hepatocellular carcinoma
2023, Biomedicine and PharmacotherapyThe detyrosination/re-tyrosination cycle of tubulin and its role and dysfunction in neurons and cardiomyocytes
2023, Seminars in Cell and Developmental BiologyHIV-1 exposure promotes PKG1-mediated phosphorylation and degradation of stathmin to increase epithelial barrier permeability
2021, Journal of Biological ChemistryMicrotubule depolymerization contributes to spontaneous neurotransmitter release in vitro
2023, Communications Biology