Extended Data Fig. 4: The extra Kir2.1 band was there but not seen previously.

a, Presence of Kir2.1 at the SR is suggested in previously published immunostaining images, as follows: i) Inter-tubular Kir2.1 staining (white arrowheads) in isolated rat cardiomyocytes (from Ponce-Balbuena et al. 2018¹); ii) Magnified images of t-tubule (left) and costamere (middle) regions stained for Kir2.1 with ß1D integrin as t-tubule marker. Representative green and red fluorescence intensity profiles along one of the white lines shown on left and middle. Note the presence of Kir2.1 bands intercalated with those of ß1D (from Sengupta et al. in 2019);² iii) Inter-tubular Kir2.1 staining (white arrowheads) in isolated rat cardiomyocytes (from Park et al. in 2020³); iv) Immunostaining (top) and traces of fluorescence intensity spatial profiles (bottom) of Kir2.1 and miR1 through ‘a’ arrow-line in immunostaining. Note the presence of one line every ≈ 1 µm (from Yang et al. in 2021)⁴. All Figures reproduced by permission of the respective authors and journals. b, Mislocalization of Adaptin in Kir2.1^Δ314-315 cardiomyocytes likely contributes to Na_V1.5 mislocalization. Confocal images show the expression pattern of Adaptin (AP1), Na_V1.5 and Kir2.1 in Kir2.1^WT (i) and Kir2.1^Δ314-315 (ii) cardiomyocytes. These images are representative of 24 cells from 3 animals analyzed.