Schwannomin-interacting Protein 1 Isoform IQCJ-SCHIP1 Is a Multipartner Ankyrin- and Spectrin-binding Protein Involved in the Organization of Nodes of Ranvier

The nodes of Ranvier are essential regions for action potential conduction in myelinated fibers. They are enriched in multimolecular complexes composed of voltage-gated Nav and Kv7 channels associated with cell adhesion molecules. Cytoskeletal proteins ankyrin-G (AnkG) and beta IV-spectrin control the organization of these complexes and provide mechanical support to the plasma membrane. IQCJ-SCHIP1 is a cytoplasmic protein present in axon initial segments and nodes of Ranvier. It interacts with AnkG and is absent from nodes and axon initial segments of beta IV-spectrin and AnkG mutant mice. Here, we show that IQCJ-SCHIP1 also interacts with beta IV-spectrin and Kv7.2/3 channels and self-associates, suggesting a scaffolding role in organizing nodal proteins. IQCJ-SCHIP1 binding requires a beta IV-spectrin-specific domain and Kv7 channel 1-5-10 calmodulin-binding motifs. We then investigate the role of IQCJ-SCHIP1 in vivo by studying peripheral myelinated fibers in Schip1 knock-out mutant mice. The major nodal proteins are normally enriched at nodes in these mice, indicating that IQCJ-SCHIP1 is not required for their nodal accumulation. However, morphometric and ultrastructural analyses show an altered shape of nodes similar to that observed in beta IV-spectrin mutant mice, revealing that IQCJ-SCHIP1 contributes to nodal membrane-associated cytoskeleton organization, likely through its interactions with the AnkG/beta IV-spectrin network. Our work reveals that IQCJ-SCHIP1 interacts with several major nodal proteins, and we suggest that it contributes to a higher organizational level of the AnkG/beta IV-spectrin network critical for node integrity.