DEPHOSPHORYLATED BUT NOT PHOSPHORYLATED MICROTUBULE-ASSOCIATED PROTEIN MAP1B BINDS TO MICROFILAMENTS

We have reported that purified native MAP1B interacts with microtubule s but not with microfilaments [Pedrotti and Islam, Cell Moth. Cytoskel . (1995) 30, 301-309]. However, MAP1B can be phosphorylated at multipl e sites by casein kinase II (CKII) and proline-directed protein kinase s (PDPK) and immunoblotting studies show that purified native MAP1B is phosphorylated at least at two CKII sites and at one PDPK site [Pedro tti et al., Biochemistry (1996) 35, 3016-3023], We now show that phosp horylation affects the in vitro binding of MAP1B with microfilaments, Native MAP1B does not bind to microfilaments but after treatment with alkaline phosphatase the dephosphorylated MAP1B binds and cosediments with microfilaments, Dephosphorylation kinetics suggest that the PDPK site, but not CKII sites, may negatively regulate the interaction with F-actin, The ability of dephosphorylated MAP1B to crosslink microfila ments was also examined and showed that MAP1B exhibits only a weak cro sslinking of F-actin when compared with MAP2.