HSPB7 is indispensable for heart development by modulating actin filament assembly

Sarcomeres, the contractile units of striated muscle, are composed of thick and thin/actin filaments. Thin filament length is closely associated with specific contractile properties of individual muscles, and it is tightly controlled by actin binding proteins. However, it is still unclear how these proteins work in concert to maintain proper thin filament length and whether there are additional factors involved. In this study, we found that deleting HSPB7 resulted in uncontrolled elongation of actin filaments and the formation of atypical actin filament bundles in cardiomyocytes. Biochemical studies revealed a previously unsuspected function of HSPB7 in interacting with and limiting actin monomer availability for actin filament polymerization, giving mechanistic insight into the etiology of aberrant sarcomeres observed in HSPB7 null heart.