IMMUNOLOGICAL EVIDENCE FOR THE LOCALIZATION OF A 110 KDA POLY(A) BINDING-PROTEIN FROM RAT-LIVER IN NUCLEAR ENVELOPES AND ITS PHOSPHORYLATION BY PROTEIN-KINASE-C

We have purified a 110 kDa poly(A) binding protein (P110) from rat liv er which is thought to be involved in mRNA translocation through the n uclear pores and have demonstrated its localisation in the nuclear env elope using polyclonal antibodies and confocal laser scanning microsco py. Although P110 was prepared from highly purified nuclear envelopes, the polyclonal antibodies raised against them bind to nucleo- and cyt oplasmic structures to a minor extent, but not to nucleolar structures . P110 decays spontaneously into several fragments which are also reco gnized by the polyclonal antibodies. The 110 kDa polypeptide and its f ragments were phosphorylated by a nuclear envelope kinase and this pho sphorylation was inhibited by a monoclonal antibody against protein ki nase C and by a specific protein kinase C inhibitor obtained from bovi ne brain. Scatchard analysis was used to determine the influence of pr otein kinase C activators and inhibitors on nuclear envelope protein p hosphorylation and RNA binding. The data indicate a close association between the RNA translocation machinery (the 110 kDa protein) and prot ein kinase C within the nuclear envelope. We suggest that the fragment ation of P110 is triggered before or during mRNA export and is not due to nonspecific proteolysis.