Mutations in MYBPC3 , the gene encoding cardiac myosin binding protein-C (cMyBP-C), have been linked to familial hypertrophic cardiomyopathy (HCM) in over 40% of patients. Therefore, cMyBP-C is a crucial regulator of a healthy heart. However, despite its importance for cardiac function, surprisingly little is still known about how cMyBP-C mediates its effects through interactions with both thick (myosin) and thin (actin) filaments (TFs). cMyBP-C binds to TFs through multiple interactions of its N-terminal domains (NTDs: C0, C1, M, and C2), which together can directly regulate cTF activation by shifting tropomyosin (Tm) from its OFF to its ON position, similar to Ca 2+ and myosin cross-bridges. All our previous knowledge about the molecular details of NTDs’ interactions with TFs comes from the helically averaged 3D reconstructions of individual NTDs bound to filamentous actin. Here, we show the first non-helical cryo-electron microscopy structure of the native cardiac TF interacting with the entire NTD of cMyBP-C. This structure reveals high-resolution molecular details of C0, C1, M, and C2 domain interactions with actin and tropomyosin. We demonstrate that C1 activates the TF through four specific salt bridges with tropomyosin and actin, leading to the activation of the tropomyosin cable. Finally, we show that cMyBP-C NTDs have a preferred mode of interaction with the native cardiac TF. Overall, our data provide a structural framework for how individual cMyBP-C domains may work in concert to activate the TF.
Risi et al. (Sun,) studied this question.