STOP proteins

Microtubules assembled from pure tubulin in vitro are labile, rapidly depol ymerized upon exposure to the cold. In contrast, in a number of cell types, cytoplasmic microtubules are stable, resistant to prolonged cold exposure. During the past years, the molecular basis of this microtubule stabilizati on in cells has been elucidated. Cold stability is due to polymer associati on with different variants of a calmodulin-regulated protein, STOP protein. The dynamic and hence the physiological consequences of STOP association w ith microtubules vary in different tissues. In neurons, STOP seems almost p ermanently associated with microtubules. STOP is apparently a major determi nant of microtubule turnover in such cells and is required for normal neuro nal differentiation. In cycling cells, only minor amounts of STOP are assoc iated with interphase microtubules and STOP does not measurably affects mic rotubule dynamics. However, STOP is associated,vith mitotic microtubules in the spindle. Recent results indicate that such an association could be vit al for meiosis and for the long-term fidelity of the mitotic process.