βIV spectrin, a new spectrin localized at axon initial segments and nodes of ranvier in the central and peripheral nervous system

We report the identification of beta IV spectrin, a novel spectrin isolated as an interactor of the receptor tyrosine phosphatase-like protein ICA512, The beta IV spectrin gene is located on human and mouse chromosomes 19q13.13 and 7b2, respectively. Alternative splicing of beta IV spectrin generates at least four distinct isoforms, numbered beta IV Sigma1-beta IV Sigma4 spectrin. The longest isoform (beta IV Sigma1 spectrin) includes an actin-binding domain, followed by 17 spectrin repeats, a specific domain in which the amino acid sequence ERQES is repeated four times, several putative SH3-binding sites and a pleckstrin homology domain. beta IV Sigma2 and beta IV Sigma3 spectrin encompass the NH2- and COOH-terminal halves of beta IV Sigma1 spectrin, respectively, while beta IV Sigma4 spectrin lacks the ERQES and the pleckstrin homology domain. Northern blots revealed an abundant expression of PIV spectrin transcripts in brain and pancreatic islets, By immunoblotting, beta IV Sigma1 spectrin is recognized as a protein of 250 kD. Anti-beta IV spectrin antibodies also react with two additional isoforms of 160 and 140 kD. These isoforms differ from beta IV Sigma1 spectrin in terms of their distribution on subcellular fractionation, detergent extractability, and phosphorylation. In islets, the immunoreactivity for beta IV spectrin is more prominent in alpha than in beta cells. In brain, beta IV spectrin is enriched in myelinated neurons, where it colocalizes with ankyrin(G) 480/270-kD at axon initial segments and nodes of Ranvier. Likewise, beta IV spectrin is concentrated at the nodes of Ranvier in the rat sciatic nerve. In the rat hippocampus, beta IV Sigma1 spectrin is detectable from embryonic day 19, concomitantly with the appearance of immunoreactivity at the initial segments. Thus, we suggest that beta IV Sigma1 spectrin interacts with ankyrin, 480/270-kD and participates in the clustering of voltage-gated Na+ channels and cell-adhesion molecules at initial segments and nodes of Ranvier.