An essential role of Tap42-associated PP2A and 2A-like phosphatases in Ty1 transcriptional silencing of S. cerevisiae.

We previously reported that overproduction of non-translatable mRNA silences Ty1 transcription, possibly via functional inactivation of the nuclear cap-binding complex (CBC) and subsequent hyperstimulation of the TORC1 pathway. Further experimental evidence for CBC-to-TORC1 signalling in Ty1 transcriptional silencing is presented here. The role of Tap42 (a key downstream component of the TORC1 pathway) was tested. Mutations affecting components of the Tap42-associated PP2A/2A-like phosphatases (Tap42, redundant Pph21/Pph22 and Sit4) eliminate Ty1 transcriptional silencing and epistasis experiments show that the phosphatases function downstream of CBC. Thus, Tap42 functions in the same positive direction as Sit4, Pph21 and Pph22 in Ty1 transcriptional silencing, providing support to the idea that Tap42 may play a positive role in phosphatase activity in response to TORC1 signalling. Moreover, the pph21-102 and sit4-102 mutations affecting interactions of the phosphatases with the Tap42 regulator also abolish Ty1 transcriptional silencing, confirming that the Tap42-phosphatase complex is one required for TORC1-mediated Ty1 transcriptional silencing. PP2A holoenzyme activity, which is independent of Tap42 and TORC1, is not essential for Ty1 transcriptional silencing, strengthening the argument that TORC1-specific signalling underlies Ty1 transcriptional silencing.

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