SMG5 Homolog, Nonsense Mediated mRNA Decay Factor (SMG5)

SMG5 interacted with a purified protein phosphatase-2A (PP2A) complex and coimmunoprecipitated with the PP2A catalytic subunit, PP2AC. Immunoprecipitates of both epitope-tagged SMG5 and endogenous SMG7 contained endogenous PP2AC. SMG5 mutants with a deletion of the conserved C-terminal region or an asp860-to-ala substitution were able to bind UPF1, but they failed to promote UPF1 dephosphorylation. The SMG5 mutants also inhibited nonsense-mediated mRNA decay in a dominant-negative manner. The deduced 1,016-amino acid protein has an N-terminal Est1 homology domain containing 2 tetratricopeptide repeat (TPR) motifs and a C-terminal PIN domain, which may be involved in nucleic acid binding or nuclease activity. A 4.6-kb EST1B transcript was ubiquitously expressed.