Abstract 14162: MAGI3 is a Novel Scaffolding Protein of the Intercalated Discs in the Heart
Proteins of the membrane associated guanylate kinase family (MAGUK) are important partners for the anchoring and scaffolding of several cardiac ion channels in plasma membrane. The protein MAGI3 is a newly discovered MAGUK protein characterized by an inverted orientation of its PDZ domains and which plays an important role in the organization of cell-cell junctions. However, its expression and role in the heart is unknown. Here we found that MAGI3 was expressed in both human and rat atrial myocardium. Immunostaining of atrial cryosections revealed that MAGI3 was exclusively localized at the level of the intercalated discs and to cell tight junctions. In co-culture system, MAGI3 was detected only at the level of myocyte-myocyte interactions and at the junctions between endothelial cells. Because MAGUK proteins are important partners for Kv channels, we studied the interactions between MAGI3 and Kv4.x channels. GST pull-down experiments showed that MAGI3 interacts with the Kv4.2 and Kv4.3 channels C-termini via the PDZ interacting motif SAL (serine-alanine-leucine). MAGI3-overexpression enhanced the Kv4.3 current recorded in whole-cell patch-clamped CHO cells (250 ± 39 pA/pF, n=10 vs control 144 ± 16 pA/pF, n=15; mean ± SEM, p<0.01) while MAGI3-silencing decreased the Kv4.3 current in HEK293 cells (116 ± 18 pA/pF, n=9 vs control 291 ± 37 pA/pF, n=10; p<0.001). These effects of MAGI3 resembled those of SAP97, another MAGUK protein (Circ Res 2009;104;758–769). We hypothesized then, that MAGI3 is a part of the Kv4.3/SAP97 complex and that these two MAGUKs cooperate for the functional expression of Kv4.3 channels. In atria, SAP97 was localized both at the intercalated discs and lateral membrane and co-localized with MAGI3 only at the intercalated discs. To determine the hierarchy of these MAGUKs in the Kv4.3 complex, we modulated MAGI3 expression to analyze Kv4.3/SAP97 interaction. GST pull-down experiments showed that inhibition of MAGI3 leads to the disruption of the Kv4.3/SAP97 interaction. In conclusion, we found a new cardiac MAGUK protein, MAGI3 which contributes to the formation of Kv4.3/SAP97 macromolecular complex. We proposed that MAGI3 determines the targeting of a sub-population of Kv4.3 to the intercalated discs.
- © 2010 by American Heart Association, Inc.