Review
The oncoprotein 18/stathmin family of microtubule destabilizers

https://doi.org/10.1016/S0955-0674(01)00289-7Get rights and content

Abstract

The past several years have seen major advances in our understanding of the mechanisms of microtubule destabilization by oncoprotein18/stathmin (Op18/stathmin) and related proteins. New structural information has clearly shown how members of the Op18/stathmin protein family bind tubulin dimers and suggests models for how these proteins stimulate catastrophe, the transition from microtubule growth to shortening. Regulation of Op18/stathmin by phosphorylation continues to capture much attention. Studies suggest that phosphorylation occurs in a localized fashion, resulting in decreased microtubule destabilizing activity near chromatin or microtubule polymer. A spatial gradient of inactive Op18/stathmin associated with chromatin or microtubules could contribute significantly to mitotic spindle assembly.

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

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