Kinetic Analysis of L1 Homophilic Interaction ROLE OF THE FIRST FOUR IMMUNOGLOBULIN DOMAINS AND IMPLICATIONSON

L1 is a cell adhesion molecule of the immunoglobulin (Ig) superfamily, critical for central nervous system development, and involved in several neuronal biological events. It is a type I membrane glycoprotein. The L1 ectodomain, composed of six Ig-like and five fibronectin (Fn) type-III domains, is involved in homophilic binding. Here, co-immunoprecipitation studies between recombinant truncated forms of human L1 expressed and purified from insect Spodoptera frugiperda Sf9 cells, and endogenous full-length L1 from human NT2N neu-rons, showed that the L1 ectodomain (L1/ECD) and L1/Ig1–4 interacted homophilically in trans, contrary to mutants L1/Ig1–3 and L1/Ig2-Fn5. All mutants were correctly folded as evaluated by combination of far-UV CD and fluorescence spectroscopy. Surface plasmon resonance analysis showed com-parable dissociation constants of 116 2 and 130 6 nM fo