The RNA binding protein Pub1 inhibits TORC1 activity in Saccharomyces cerevisiae

Pub1 is an RNA binding protein with multiple functions in gene expression, including the control of translation and mRNA stability. It also acts as a prion-like protein given its ability to self-aggregate, and it plays a role in the formation of mRNA-protein assemblies such as stress granules. This study identifies an unexpected relationship between Pub1 and the nutrient signalling complex TORC1. Whereas Pub1-deficiency increased cellular resistance to TORC1 inhibition by rapamycin, PUB1 overexpression impaired growth, particularly when nutrients were scarce. This growth defect was suppressed by TOR1 kinase deletion, which indicates that Pub1 functions are channelled through the TORC1 complex. Moreover, Pub1 antagonised TORC1-regulated processes, such as autophagy and the phosphorylation of ribosomal protein Rps6. We found that Pub1 was required for the aggregation of Kog1, a component of the TORC1 complex upon starvation, whereas its overexpression caused Kog1 foci formation in non-stressed growing cells. All these results suggest a negative role of Pub1 in regulating TORC1 activity based on its ability to promote the formation of TORC1-ribonucleoprotein aggregates.