Regulation of the cochaperone BAG3 and chaperone-assisted selective autophagy by Hippo kinases

• Applicant: Professor Dr. Jörg Höhfeld
• Subject Area: Cell Biology
• Term: from 2015 to 2019
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 279436271

The cochaperone BAG3 (BCL2-associated athanogene-3) is a key proteostasis factor in muscle, immune and neuronal cells. In addition, its overexpression contributes to oncogenic transformation. BAG3 induces a chaperone-assisted and ubiquitin-dependent autophagy pathway, called CASA (chaperone-assisted selective autophagy). The pathway mediates the lysosomal degradation of nonnative chaperone clients, such as the actin-anchoring protein filamin following its mechanical unfolding in contracting muscles and adherent cells. Besides its degradation-inducing activity, BAG3 also regulates protein synthesis. The cochaperone interacts with components of the Hippo signaling pathway to facilitate filamin expression. The BAG3-mediated coregulation of protein degradation and protein synthesis is essential for maintaining the actin cytoskeleton under mechanical strain. Here we propose to analyze the impact of Hippo signaling on BAG3 function and CASA activity. The project is based on our identification of multiple Hippo kinases in association with BAG3. Our preliminary data show that at least one of these kinases directly controls CASA activity in smooth muscle cells. We will apply biochemical and cell biological assays to verify functional interactions between Hippo kinases and the CASA machinery in muscle and neuronal cells. Phosphoproteomics will be performed to identify Hippo phosphorylation sites in CASA components, and their importance for BAG3 and CASA regulation will be explored. Finally, differentiated skeletal muscle cells, isolated mouse muscles and transgenic D. melanogaster lines will be used to investigate the role of Hippo signaling in BAG3-mediated muscle maintenance. The work should establish key mechanisms for the regulation of protein homeostasis in eukaryotes, relevant for our understanding of muscle weaknesses, neurodegeneration and cancer.

• DFG Programme: Research Grants