Figure 2. Casq2^–/– hearts lack calsequestrin, display no apparent upregulation of other SR Ca²⁺-binding proteins, and have decreased triadin 1 and junctin protein levels.

(A) Forty micrograms of homogenate protein from Casq2^+/+, Casq2^+/–, and Casq2^–/– hearts and 30 μg of microsomal protein from control membranes from mouse heart (Casq2) and skeletal muscle (Casq1) were electrophoresed per lane and probed with anti-calsequestrin antibody. Cardiac (Casq2), skeletal muscle (Casq1), and Casq-like proteins are indicated. (B) ⁴⁵Ca²⁺ overlay and Stains-all staining of SR membrane proteins obtained from Casq2^+/+, Casq2^+/–, and Casq2^–/– hearts. Seventy-five micrograms of SR membrane protein was loaded per lane in duplicate and subjected to SDS-PAGE, then one-half of the gel was processed for ⁴⁵Ca²⁺ overlay (left) and the other half stained with Stains-all (right). One microgram of purified canine Casq2 was also run as an internal standard. (C) Immunoblot detection of SR proteins in microsomes isolated from 10 Casq2^+/+, 10 Casq2^+/–, and 10 Casq2^–/– hearts. Forty micrograms of microsomal protein were electrophoresed per lane, transferred to nitrocellulose paper, and probed with the antibodies indicated on the left. (D) Quantification of protein expression levels. Data represent average values for 4 hearts per genotype expressed relative to Casq2^+/+ values. RyR2, cardiac isoform of the RyR; SER, SERCA2a or cardiac isoform of the Ca²⁺ pump; TRN, triadin 1 or major cardiac isoform of triadin; JCT, junctin; *P < 0.05.