Figure 6.

The MT-dependent phosphorylating activity cosediments with MTs. Phosphorylation of human stathmin/Op18 in total (T), supernatant (S), and pellet (P) of high-speed mitotic Xenopus egg extracts. Extract aliquots were incubated for 45 min with 5% DMSO alone (+) or with 10 μM NZ (+NZ), then centrifuged to separate MTs from soluble tubulin. [γ-³²P] ATP and recombinant human wild-type stathmin/Op18 were added to equal volumes of total, supernatant and resuspended pellet fractions which were incubated for 15 min at room temperature (control samples with no human stathmin/Op18 added are indicated by −). Tubulin was revealed by immunodetection. Human stathmin/Op18 was either specifically revealed by antiserum C (which does not cross-react with Xenopus stathmin/Op18), or by autoradiography (³²P). Arrows point to hyperphosphorylated human forms “16 ” and “17.”