Obscurins are giant cytoskeletal proteins with structural and regulatory roles. Obscurin-B (~870 kDa), the largest known isoform, contains 2 enzymatically active Ser/Thr kinase (kin) domains, kin1 and kin2, which belong to the myosin light chain kinase family. Kin1 binds to and phosphorylates N-cadherin, a major component of the intercalated disc, the unique sarcolemmal microdomain that mediates the mechanochemical coupling of adjacent cardiomyocytes. Obscurin-B containing kin1 and N-cadherin colocalize at cell junctions in embryonic rat ventricular myocytes (ERVMs), and their codistribution is regulated by Ca ²⁺ . Phosphoproteomics analysis revealed that obscurin-kin1 phosphorylates N-cadherin at Ser-788 located within the juxtamembrane region of its cytoplasmic domain, with an apparent K _cat of approximately 5.05 min ^–1 . Overexpression of obscurin-kin1 or phosphomimic-Ser-788-Glu N-cadherin in ERVMs markedly increases cell adhesion and chemical coupling. Importantly, phosphomimic Ser-788-Glu N-cadherin exhibits significantly reduced binding to p120-catenin, while overexpression of phosphoablated Ser-788-Ala N-cadherin increases RhoA activity. Consistent with an essential role of the obscurin-kin1/N-cadherin axis in cardiomyocyte coupling, it is deregulated in end-stage human heart failure. Given the nearly ubiquitous expression of obscurin and N-cadherin, our findings may have broad applicability in deciphering the obscurin-kin1/N-cadherin axis that likely mediates cell coupling in diverse tissues and organs.

Myosin-binding protein C (MyBP-C) comprises a family of myofilament proteins that maintain sarcomeric structure and regulate actomyosin crossbridge cycling. Pathogenic variants in MYBPC1, the gene encoding the slow skeletal isoform (sMyBP-C), lead to a dominant congenital myopathy, termed Myotrem, characterized by muscle weakness, hypotonia, and a distinctive tremor of myogenic origin, in the absence of neuropathy. However, the molecular mechanism(s) of myogenic tremorgenesis is largely unknown. One potential mechanism is aberrant myofilament stretch activation, which is defined as a delayed increase in force after a rapid stretch. We utilized the Myotrem murine model harboring the pathogenic MYBPC1 E248K variant to test the hypothesis that stretch activation is augmented in permeabilized Myotrem E248K soleus fibers. We found that stretch activation was significantly increased in E248K soleus muscle fibers. Interestingly, once submaximally Ca²⁺ activated, a subpopulation of slow-twitch E248K fibers exhibited spontaneous pulsatile sarcomere oscillations. This pulsing behavior generated a sinusoidal waveform pattern in sarcomere length, which often persisted on a timescale of minutes. These results align with sMyBP-C as key regulator of the synchronous activation of myofilaments by dampening both spontaneous oscillatory activity and stretch-dependent activation. We propose that the presence of sMyBP-C-E248K disrupts this regulation, thereby driving pathogenic myogenic tremors.