The growth-related, translationally controlled protein P23 has properties of a tubulin binding protein and associates transiently with microtubules during the cell cycle

The translationally controlled protein P23 was discovered by the early indu ction of its rate of synthesis after mitogenic stimulation of mouse fibrobl asts. P23 is expressed in almost all mammalian tissues and it is highly con served between animals, plants and yeast. Based on its amino acid sequence, P23 cannot be attributed to any known protein family, and its cellular fun ction remains to be elucidated. Here, we present evidence that P23 has prop erties of a tubulin binding protein that associates with microtubules in a cell cycle-dependent manner. (1) P23 is a cytoplasmic protein that occurs i n complexes of 100-150 kDa, and part of P23 can be immunoprecipitated from HeLa cell extracts with anti-tubulin antibodies, (2) In immunolocalisation experiments we find P23 associated with microtubules during G(1), S, G(2) a nd early M phase of the cell cycle. At metaphase, P23 is also bound to the mitotic spindle, and it is detached from the spindle during metaphase-anaph ase transition. (3) A GST-P23 fusion protein interacts with alpha- and beta -tubulin, and recombinant P23 binds to taxol-stabilised microtubules in vit ro. The tubulin binding domain of P23 was identified by mutational analysis ; it shows similarity to part of the tubulin binding domain of the microtub ule-associated protein MAP-1B, (4) Overexpression of P23 results in cell gr owth retardation and in alterations of cell morphology. Moreover, elevation of P23 levels leads to microtubule rearrangements and to an increase in mi crotubule mass and stability.