Modulation of Ca2+-induced Ca2+ release by ubiquitin protein ligase E3 component n-recognin UBR3 and 6 in cardiac myocytes.

Ca2+-induced Ca2+ release (CICR) from sarcoplasmic reticulum is a finely tuned process responsible for cardiac excitation and contraction. The ubiquitin-proteasome system (UPS) as a major degradative system plays a crucial role in the maintenance of Ca2+ homeostasis. The E3 component N-recognin (UBR) subfamily is a part of the UPS; however, the role of UBR in regulating cardiac CICR is unknown. In the present study, we found that among the UBR family, single knockdown of UBR3 or UBR6 significantly elevated the amplitude of sarcoplasmic reticulum Ca2+ release without affecting Ca2+ transient decay time in neonatal rat ventricular myocytes. The protein expression of alpha 1 C subunit of L-type voltage-dependent Ca2+ channel (Cav1.2) was increased after UBR3/6 knockdown, whereas the protein levels of RyR2, SERCA2a, and PLB remained unchanged. In line with the increase in Cav1.2 proteins, the UBR3/6 knockdown enhanced the current of Cav1.2 channels. Furthermore, the increase in Cav1.2 proteins caused by UBR3/6 reduction was not counteracted by a protein biosynthesis inhibitor, cycloheximide, suggesting a degradative regulation of UBR3/6 on Cav1.2 channels. Our results indicate that UBR3/6 modulates cardiac CICR via targeting Cav1.2 protein degradation.