Amplified In Breast Cancer 2 (ABC2)

The kinase SMG1 forms a surveillance complex with UPF1 (RENT1), ERF1 (ETF1), and ERF3 (GSPT1) on spliced mRNAs during recognition of premature termination codons. If an exon junction complex (EJC) exists downstream from the surveillance complex, SMG1 phosphorylates UPF1, and this step is rate limiting for NMD. Both SMG8 and SMG9 suppressed the kinase activity of SMG1, and both were SMG1 target substrates.

Depletion of SMG9 resulted in accumulation of phosphorylated UPF1. SMG8 was required to recruit SMG1 to the surveillance complex, and depletion of SMG8 caused accumulation of a complex containing the ribosome, UPF1, ERF1, ERF3, and the EJC on the spliced mRNA-protein complex.