The possible significance of adenosine 3':5'-cyclic monophosphate-stimulated protein kinase activity associated with purified microtubular protein preparations from mammalian brain

The phosphorylation of brain microtubular proteins in situ and in vitro

The phosphorylation of microtubular proteins isolated by reassembly in vitro from slices of guinea-pig cerebral cortex labelled with [32P]orthophosphate was investigated. Under the conditions tested, both and the alpha and beta forms of tubulin contained metabolically-active P which accounted for about one third of the total 32P incorporated into protein; the remaining protein-bound 32P was associated with 3-4 minor high MW components co-purifying with tubulin during two cycles of assembly-disassembly. Microtubular protein prepared in this way contained approx. 0.8 mol of alkalilabile P/mol of tubulin dimer (M.W. 110,000). In vitro studies showed that reassembled microtubular protein preparations catalysed the incorporation of up to 0.55 mol of P/mol of tubulin dimer during incubation with Mg2+ and [gamma 32P]ATP. The reaction was linear during the first 30 min of incubation at 37 degrees C. Cyclic AMP (10 microM, final concentration) caused a transient increase in the initial rates of tubulin phosphorylation. Little label was incorporated into the minor high M.W. components under these conditions. The in vitro phosphorylation of microtubular protein increased in a non-linear manner with respect to protein concentration: this was in contrast to earlier experiments showing linear kinetics when chromatographically isolated tubulin was tested for intrinsic kinase activity. Isolated microtubular protein preparations bound [3H]GTP, [3H]ATP and to a lesser extent, [3H]cyclic AMP, and exhibited Ca(2+)-ATPase activity (up to 60 pmol Pi released min/mg protein at 37 degrees C).

Cyclic AMP-dependent endogenous phosphorylation of a microtubule-associated protein

Microtubules prepared from chick brain homogenates by successive cycles of assembly-disassembly were found to contain two high-molecular-weight proteins, designated microtubule-associated protein1 and microtubule-associated protein2. Microtubule-associated protein2 (apparent molecular weight 300,000 by sodium dodecyl sulfate-polyacrylamide gel electrophoresis) was the preferred substrate for an endogenous cyclic AMP-dependent protein kinase which appeared to be an integral component of the microtubules. The initial rate of phosphorylation of microtubule-associated protein2 was enhanced 4- to 6-fold by cyclic AMP, with half-maximal stimulation occurring at 2 times 10-7 M cyclic AMP. Under optimal conditions, a total of 1.0 and 1.9 mol of phosphate was incorporated per mole of microtubule-associated protein2, in the absence and presence of cyclic AMP, respectively. Cyclic AMP also stimulated the phosphorylation of tubulin, but the rate of phosphate incorporation per mol of tubulin was only 0.15% that of microtubule-associated protein2. The data raise the possibility that the cyclic AMP-dependent phosphorylation of microtubule-associated protein 2 may play a role in microtubule assembly or function.