Binding of polyribonucleotides and polydeoxyribonucleotides to bovine brain microtubule protein: Age-dependent modulation via phosphorylation of high-molecular-weight microtubule-associated proteins and tau proteins

Abstract

Binding of both synthetic poly(A) and naturally occurring poly(A)(+)mRNA as well as DNA to microtubule protein is mediated by microtubule-associated proteins; tubulin itself is not capable of binding these polymers. Bovine brain microtubule protein from immature animals was found to have a significantly lower capacity to bind poly(A) than microtubule protein from old animals. On the other hand, “old” microtubule protein binds DNA more efficiently than “immature” microtubule protein. Microtubule-associated protein 2 [preferred binding site for DNA] and tau proteins [preferred binding site for poly (A)] are specifically phosphorylated by a microtubule-associated, cAMP-dependent protein kinase. It was found that the affinity of microtubule protein for poly(A) is markedly decreased by autophosphorylation of the protein; in the case of DNA, the decrease in affinity was less. Autophosphorylation of “immature” microtubule proteins diminished the binding capacity for poly(A) to a greater extent than do “old” proteins. Scatchard plot analysis revealed that microtubule-protein possesses two different binding sites for poly(A). The corresponding dissociation constants were found to be increased in the phosphorylated system, but phosphorylation does not appear to alter the total number of binding sites. Compared to immature animals, microtubule protein from “old” bovine brains was found to have a reduced number of binding sites for poly(A), whereas the values of the dissociation constants remain unchanged. In contrast to total microtubule protein and homogeneous microtubule-associated protein 2, only one kind of binding site for poly(A) could be detected in homogeneous tau protein. No influence of different RNA or DNA species on microtubule protein-associated cAMP-dependent protein kinase, adenosine triphosphatase and guanosine triphosphatase activities could be detected.