IMPLICATIONS FOR THE DOMAIN ARRANGEMENT OF AXONIN-1 DERIVED FROM THE MAPPING OF ITS NGCAM BINDING SITE

摘要

The neuronal cell adhesion molecule axonin-1 is composed of six immunoglobulin and four fibronectin type III domains, Axonin-1 promotes neurite outgrowth, when presented as a substratum for neurons in vitro, via a neuronal receptor that has been identified as the neuron-glia cell adhesion molecule, NgCAM, based on the blocking effect of polyclonal antibodies directed to NgCAM, Here we report the identification of axonin-1 domains involved in NgCAM binding, NgCAM-conjugated microspheres were tested for binding to COS cells expressing domain deletion mutants of axonin-1, In addition, monoclonal antibodies directed to axonin-1 were assessed for their ability to block the axonin-1-NgCAM interaction, and their epitopes were mapped using the domain deletion mutants, The results suggest that the four amino-terminal immunoglobulin domains of axonin-1 form a domain conglomerate which is necessary and sufficient for NgCAM binding, Surprisingly, NgCAM binding to membrane-bound axonin-1 was increased strongly by deletion of the fifth or sixth immunoglobulin domains of axonin-1, Based on these results and on negative staining electron microscopy, we propose a horseshoe-shaped domain arrangement of axonin-1 that obscures the NgCAM binding site, Neurite outgrowth studies with truncated forms of axonin-1 show that axonin-1 is a neurite outgrowth-promoting substratum in the absence of the NgCAM binding site.