Stathmin/Op 18 is a microtubule (MT) dynamics-regulating protein that has b
een shown to have both catastrophe-promoting and tubulin-sequestering activ
ities. The level of stathmin/Op18 phosphorylation was proved both in vitro
and in vivo to be important in modulating its MT-destabilizing activity. To
understand the in vivo regulation of stathmin/Op18 activity, we investigat
ed whether MT assembly itself could control phosphorylation of stathmin/Op1
8 and thus its MT-destabilizing activity. We found that MT nucleation by ce
ntrosomes from Xenopus sperm or somatic cells and MT assembly promoted by d
imethyl sulfoxide or paclitaxel induced stathmin/Op18 hyperphosphorylation.
in Xenopus egg extracts, leading to new stathmin/Op18 isoforms phosphoryla
ted on Ser 16. The MT-dependent phosphorylation of stathmin/Op18 took place
in interphase extracts as well, and was also observed in somatic cells. We
show that the MT-dependent phosphorylation of stathmin/Op18 on Ser 16 is m
ediated by an activity associated to the MTs, and that it is responsible fo
r the stathmin/Op18 hyperphosphorylation reported to be induced by the addi
tion of "mitotic chromatin." Our results suggest the existence of a positiv
e feedback loop, which could represent a novel mechanism contributing to MT
network control.