The 130- and 160-kD polypeptide forms of the neural cell adhesion molecule (NCAM) were analyzed by electron microscopy after low angle rotary shadowing and freeze replication. Individual NCAM molecules appeared as uniformly thick rods, with a distinct bend or hinge region near their middle. Aggregates were also present, containing two to six rods in a pinwheel-like configuration without measurable overlap between rods. The 130- and 160-kD NCAM forms had lengths of 38 and 51 nm, respectively, with a difference in arm length distal to the bend, but not toward the center of the pinwheel. Although enzymatic removal of the polysialic acid moiety on NCAM did not alter the appearance of individual molecules, it did increase the average number of arms per aggregate. Monoclonal antibodies that recognize defined regions of the NCAM polypeptide were used to provide landmarks on the observed molecular figures. Two antibodies specific for cytoplasmic epitopes near the COOH terminus were clustered at the distal tip of aggregated arms. Two other antibodies that react with epitopes near the NH2 terminus and the middle of the molecule bound to sites more centrally located on the pinwheel structure. Together, these results suggest that the observed aggregates represent an association of molecules near their NH2-terminal homophilic binding site, and have led to several predictions about the nature of an NCAM-mediated cell-cell bond.

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