Characterization of the L1-neurocan-binding site - Implications for L1-L1 homophilic binding

The L1 adhesion molecule is a 200-220-kDa membrane glycoprotein of the Ig s uperfamily implicated in important neural processes including neuronal cell migration, axon outgrowth, learning, and memory formation. L1 supports hem ophilic L1-L1 binding that involves several Ig domains but can also bind wi th high affinity to the proteoglycan neurocan, it has been reported that ne urocan can block hemophilic binding; however, the mechanism of inhibition a nd the precise binding sites in both molecules have not been determined. By using fusion proteins, site-directed mutagenesis, and peptide blocking exp eriments, we have characterized the neurocan-binding site in the first Ig-l ike domain of human L1, Results from molecular modeling suggest that the se quences involved in neurocan binding are localized on the surface of the fi rst Ig domain and largely overlap with the G-F-C beta -strands proposed to interact with the fourth Ig domain during hemophilic binding. This suggests that neurocan may sterically hinder a proper alignment of LI domains. We f ind that the C-terminal portion of neurocan is sufficient to mediate bindin g to the first Ig domain of L1, and we suggest that the sushi domain cooper ates with a glycosaminoglycan side chain in forming the binding site for L1 .