Phosphatase activity of small C-terminal domain phosphatase 1 (SCP1) controls the stability of the key neuronal regulator RE1-silencing transcription factor (REST)

The RE1-silencing transcription factor (REST) is the major scaffold protein for assembly of neuronal gene silencing complexes that suppress gene transcription through regulating the surrounding chromatin structure. REST represses neuronal gene expression in stem cells and non-neuronal cells, but it is minimally expressed in neuronal cells to ensure proper neuronal development. Dysregulation of REST function has been implicated in several cancers and neurological diseases. Modulating REST gene silencing is challenging because cellular and developmental differences can affect its activity. We therefore considered the possibility of modulating REST activity through its regulatory proteins. The human small C-terminal domain phosphatase 1 (SCP1) regulates the phosphorylation state of REST at sites that function as REST degradation checkpoints. Using kinetic analysis and direct visualization with X-ray crystallography, we show that SCP1 dephosphorylates two degron phosphosites of REST with a clear preference for phosphoserine 861 (pSer-861). Furthermore, we show that SCP1 stabilizes REST protein levels, which sustains REST's gene silencing function in HEK293 cells. In summary, our findings strongly suggest that REST is a bona fide substrate for SCP1 in vivo and that SCP1 phosphatase activity protects REST against degradation. These observations indicate that targeting REST via its regulatory protein SCP1 can modulate its activity and alter signaling in this essential developmental pathway.