Regulation of Hippo pathway by Hsp70-Bag3 complex: Bag3 modulates YAP phosphorylation and its nuclear translocation.

Protein abnormalities can accelerate aging causing protein misfolding diseases, various adaptive responses have evolved to relieve proteotoxicity. To trigger these responses, cells must detect the buildup of aberrant proteins. Previously we demonstrated that the Hsp70-Bag3 (HB) complex senses the accumulation of defective ribosomal products, stimulating signaling pathways, such as stress kinases or the Hippo pathway kinase LATS1. Here, we studied how Bag3 regulates the ability for LATS1 to regulate its key downstream target YAP. In naive cells, Bag3 recruited a complex of LATS1, YAP, and the scaffold AmotL2, which links LATS1 and YAP. Upon inhibition of proteasome, AmotL2 dissociated from Bag3, which prevented phosphorylation of YAP by LATS1 and led to consequent nuclear YAP localization together with Bag3. Mutations in Bag3 that enhanced its translocation into nucleus, also facilitated nuclear translocation of YAP. Interestingly, Bag3 also controlled YAP nuclear localization in response to cell density, indicating broader roles beyond proteotoxic signaling responses for Bag3 in the regulation of YAP. These data implicate Bag3 as a regulator of Hippo pathway signaling, and suggest mechanisms by which proteotoxic stress signals are propagated.