Studies on the phosphorylation of brain microtubular components

Microtubular protein isolated from brains of adult rats or 1 to 3-day old chicks, previously injected with 32P, contained considerable amounts of serine-bound 32P measured as alkali-labile protein bound 32P. Between 40 and 80% of the acid-stable tubulin-bound 32P was extractable with lipid solvents and was shown to be associated with several major phospholipid classes. Experiments in which tubulin purified by poly-merisation was incubated with [gamma-32P] ATP revealed that tubulin itself acted as a substrate for a microtubule-associated protein kinase under in vitro polymerisation conditions. Further investigations concerning the biochemical properties of brain microtubulin fractions prepared in the presence or absence of glycerol revealed that glycerol brought about a re-distribution of total protein and of phospholipid-and protein-bound 32P. In a final series of experiments, the interactions between myo-inositol and purified brain tubulin were investigated. It was found that myo-inositol binds to tubulin (0.5--1.0 moles of myo-inositol/mole of tubulin dimer) and that myo-inositol affected the stability of in vitro polymerised microtubules. The data presented in this work provide new biochemical evidence for interactions between microtubules and membranes, the functional implications of which are discussed.<p

L-glutamate and phorbol ester stimulate the release of secretory amyloid precursor protein from rat cortical synaptosomes

Treatment of rat cortical synaptosomes with micromolar concentrations of L-glutamate stimulated the release of the secreted form of amyloid precursor protein in a concentration-dependent, however biphasic manner as assayed by semiquantitative Western blot analysis. The secreted amyloid precursor protein released from synaptosomes into the incubation medium was highest in the presence of 500 µM L-glutamate (about 64% over the level assayed in the incubation medium in the absence of any drug). In contrast, direct stimulation of protein kinase C by phorbol-12-myristate-13-acetate resulted in a concentration-independent increase in secretory amyloid precursor protein release by about 100% already detectable at a concentration of 0.1 µM but with no significant change at higher concentrations up to 10 µM. The presented data show that there is a constitutive release of secretory amyloid precursor protein from synaptosomes and suggest that (i) processing of amyloid precursor protein at the synaptic level is controlled by L-glutamate presumably via activation of protein kinase C, and (ii) isolated cortical synaptosomes represent a useful experimental approach to selectively study amyloid precursor protein metabolism at the synaptic level