N-Terminal Domains of Cardiac Myosin Binding Protein C Cooperatively Activate the Thin Filament

Abstract

SUMMARY Muscle contraction relies on interaction between myosin-based thick filaments and actin-based thin filaments. Myosin binding protein-C (MyBP-C) is a key regulator of actomyosin interactions. Recent studies established that the N’-terminal domains (NTDs) of MyBP-C can either activate or inhibit thin filaments, but the mechanism of their collective action is poorly understood. Cardiac MyBP-C (cMyBP-C) harbors an extra NTD which is absent in skeletal isoforms of MyBP-C and its role in regulation of cardiac contraction is unknown. Here we show that the first two domains of human cMyPB-C (i.e., C0 and C1) cooperate to activate the thin filament. We demonstrate that C1 interacts with tropomyosin via a positively charged loop and that this interaction, stabilized by the C0 domain, is required for thin filament activation by cMyBP-C. Our data reveal a mechanism by which cMyBP-C can modulate cardiac contraction and demonstrate a function of the C0 domain.

Thin filament dysfunctions caused by mutations in tropomyosin Tpm3.12 and Tpm1.1

“…Differing tropomyosin positions are the basis of the classic steric-blocking mechanism of muscle contraction regulation, in which tropomyosin blocks myosin-binding sites on actin (B-state), partially reveals myosin-binding sites (C-state), or does not interfere with myosin binding (M-state) (Lehman 2016). Because tropomyosin covers the entire thin filament, the positions it assumes might also be important for actin interactions with nebulin or myosinbinding protein C, a possibility that has recently gained experimental support (Lin et al 2018;Marttila et al 2014a;Risi et al 2018).…”

Infantile restrictive cardiomyopathy: cTnI-R170G/W impair the interplay of sarcomeric proteins and the integrity of thin filaments

“…This event leads to a series of conformational changes: the regulatory C-terminal region of cTnI is released from tropomyosin/actin, allowing after an azimuthal movement of tropomyosin the actin-myosin interaction. In addition, myosin binding protein C (cMyBPC), anchored to the thick filament, may also bind to actin with its N-terminus (domains C0-C2) to support the shift of tropomyosin to a more active state of the filament at low Ca 2+ [1,2]. The functional interplay of cMyBPC and troponin has been described before [3].…”

MyBP-C: one protein to govern them all

“…MyBP-C's interaction with actin was discovered shortly after its ability to bind myosin was determined (Moos et al 1978). Since then, numerous experimental approaches have collectively shown the N-terminal region of MyBP-C is responsible for these interactions (Belknap et al 2014;Harris et al 2016;Inchingolo et al 2019;Kensler et al 2011;Lu et al 2011;Luther et al 2011;Mun et al 2011Mun et al , 2014Orlova et al 2011;Risi et al 2018;Whitten et al 2008). The N-terminal domains possess some interesting structural attributes; the Ig-like C0 domain is specific to the cardiac isoform and the proline-alanine rich region between C0 and C1 has sequences proposed to bind actin (Squire et al 2003), that have been linked to modulating contractile velocity across species .…”

“…However, in these studies, despite visualizing the tropomyosin shift, cMyBP-C was not observed as a clear density instead only the proximal region was visible. Recently, cryo-EM studies were able to visualize the whole of the N-terminal fragment bound to the thin filament (Risi et al 2018). This difference may be due to the lower levels of decoration used in the negative stain experiments resulting in loss of apparent density due to averaging, or could reflect a dynamic interaction between cMyBP-C and the thin filament; the latter has recently been directly observed (Inchingolo et al 2019).…”