The contactin-related protein FAR-2 defines Purkinje cell clusters and labels subpopulations of climbing fibers in the developing cerebellum

FAR-2 is a novel neural member of the Ig superfamily, which is related to F 11/F3/contactin and axonin-1/TAG-1. This protein is expressed by subpopulat ions of Purkinje cells in the chicken cerebellum and FAR-2-positive cluster s of these neurons alternate with FAR-2-negative clusters in both tangentia l dimensions of the cerebellar cortex. Furthermore, FAR-2 is also expressed by one type of Purkinje cell afferents, namely, the climbing fibers, and d ifferent subpopulations of these axons show distinct levels of FAR-2 expres sion. Homology modeling using axonin-1 as a template reveals that the four aminoterminal Ig domains of FAR-2 form a compact U-shaped structure, which is likely to contain functionally important ligand-binding sites. FAR-2 is binding to the Ig superfamily protein NgCAM/L1, but not to the related rece ptor NrCAM, and it is also interacting with the modular ECM protein tenasci n-R. These results suggest that FAR-2 may contribute to the formation of so matotopic maps of cerebellar afferents during the development of the nervou s system.