Cardiac stress leads to regulation of Filamin C dimerisation via an ancient phosphorylation-modulated interaction with HSPB7

The biomechanical properties and responses of tissues underpin a variety of physiological functions and pathologies. In striated muscle, the actin-binding protein filamin C (FLNC) is a key protein whose variants causative for a wide range of cardiomyopathies and musculoskeletal pathologies. Seemingly a multi-functional protein that interacts with a variety of partners, how FLNC is regulated at the molecular level is not well understood. Here we have investigated its interaction with HSPB7, a cardiac-specific molecular chaperone whose absence is embryonically lethal. We found that FLNC and HSPB7 interact in cardiac tissue under biomechanical stress, forming a strong hetero-dimer whose structure we have solved by means of X-ray crystallography. Our quantitative analyses show that the hetero-dimer out-competes the FLNC homo-dimer interface, potentially acting to abrogate the ability of the protein to cross-link the actin cytoskeleton, and to enhance its diffusive mobility. We show that phosphorylation of FLNC at threonine 2677, located at the dimer interface and associated with cardiac stress, acts to favour the homo-dimer. Conversely, phosphorylation at tyrosine 2683, also at the dimer interface, has the opposite effect and shifts the equilibrium towards the hetero-dimer. Evolutionary analysis and ancestral sequence reconstruction reveals this interaction and its mechanisms of regulation to date around the time primitive hearts evolved in chordates. Our work rationalises on the molecular level how FLNC might switch between stabilising functions in the cell, and reveals how HSPB7 acts as a specific molecular chaperone that regulates FLNC. ### Competing Interest Statement The authors have declared no competing interest.