Residue S368 of Cx43 is phosphorylated in Rin cells expressing Cx43-WT, Cx43-CK1-A or Cx43-CK1-D. (A,B,D,E,G,H) Fluorescent (upper) and fluorescent merged with the corresponding differential interference contrast (DIC, lower) images of cells stained with a Cx43-pS368 phospho-specific antibody (pS368). Cx43-pS368 (green) was found in junctional plaques in groups and isolated pairs of Cx43WT (A,B), Cx43-CK1-A (D,E) and Cx43-CK1-D (G,H) cells. (C,F,I) DIC and fluorescent images of cells stained simultaneously with a polyclonal Cx43 antibody (“total” Cx43, tCx43) and a phospho-specific antibody against Cx43-pCK1 (pCK1). In all groups, total Cx43 (green) was found in junctional plaques and other cell areas (C,F,I, upper left panels). Bona fide CK1-phosphorylated Cx43 (red) was found only in Cx43WT cells (C, bottom right) colocalized with a membrane fraction of total Cx43 at junctional plaques (C, upper right, yellow). The pCK1 antibody labels a non-specific nuclear signal in the dephospho- (F) and phospho-mimicking (I) mutant expressing cells that is also found in parental Rin cells, devoid of connexins (Figure S5). Calibration bars (pink lines, 25 μm) apply to each column.