Eukaryotic translation termination factor 1 associates with protein phosphatase 2A and targets it to ribosomes

Purification of type 2A protein phosphatase (PP2A) from rabbit skeletal muscle resulted in the isolation of a trimeric phosphatase which is composed of a catalytic (PP2Ac), a structural (PR65 alpha/A alpha), and a regulatory (PR55 alpha/B alpha) subunit, together with translation termination factor 1 (eRF1) and another protein of 55 kD (EMBO J., 15, 101-112). Yeast two-hybrid system analysis demonstrated that the eRF1 interacted with PP2Ac alpha but not with PR65 alpha/A alpha or PR55 alpha/B alpha. The N-terminal region of PP2Ac alpha, comprising 50 amino acid residues, and the C-terminal part of eRFI, corresponding to an internal region between amino acids 338-381, were found to be necessary for eRF1-PP2Ac alpha interaction in yeast. Immunoprecipitations using 12CA5 antibodies and extracts from COS1 cells transiently transfected with eRFI tagged with 9-amino acid epitope from influenza hemagglutinin (HA) demonstrated the presence of eRF1-PP2Ac alpha-PR65 alpha/A alpha complex in these cells. In addition, polysomes obtained from COS1 cells overexpressing HA-eRFI displayed several-fold higher PP2A activity than control polysomes. No effect of either PP2Ac or dimeric and trimeric PP2A holoenzymes on the rate of translation termination was detected using an in vitro reconstituted translation termination assay. In summary, eRFI appears to represent a novel PP2A-targeting subunit that brings this phosphatase in contact with putative ribosomal substrate(s). It remains to be established whether termination of translation requires dephosphorylation of participating protein factor(s).