Abstract. The NG2 chondroitin sulfate proteoglycan is a membrane-associated molecule of ,o500 kD with a core glycoprotein of 300 kD. Both the complete proteoglycan and a smaller quantity of the 300-kD core are immunoprecipitable with polyclonal and monoclonal antibodies against purified NG2. From some cell lines, the antibodies coprecipitate NG2 and type VI collagen, the latter appearing on SDS-PAGE as components of 140 and 250 kD under reducing conditions. The immunoprecipitation of type VI collagen does not seem to be due to recognition of the collagen by the antibodies, but rather to binding of the collagen to NG2. Studies on the NG2-type VI collagen complex suggest that binding between the two molecules is mediated by protein-protein interactions rather than by ionic interactions involving the glycosaminoglycans.Immunofluorescence double labeling in frozen sections of embryonic rat shows that NG2 and type VI collagen are colocalized in structures such as the intervertebral discs and arteries of the spinal column. In vitro the two molecules are highly colocalized on the surface of several cell lines. Treatment of these cells resulting in a change in the distribution of NG2 on the cell surface also causes a parallel change in type VI collagen distribution. Our results suggest that cell surface NG2 may mediate cellular interactions with the extraceUular matrix by binding to type VI collagen.

The NILE glycoprotein is a rat neuronal cell adhesion molecule which has been reported to be very similar in structure, function, and distribution to the mouse L1 glycoprotein. Here we report the complete nucleotide sequence of the NILE message (5,208 nucleotides) and the deduced amino acid sequence of the NILE polypeptide (1,257 amino acids). The predicted NILE protein is 96% identical to L1 at the amino acid level, confirming that the two molecules are homologues. The sequence information shows that NILE is a transmembrane molecule with an extensive ectodomain and a much smaller cytoplasmic domain. The extracellular portion of the molecule contains six immunoglobulin C-2 type domains followed by five fibronectin type III repeats. These two structural motifs are characteristic of several other cell adhesion molecules. The cytoplasmic tails of NILE and L1 are identical to each other and distinct from the cytoplasmic regions of all other cell adhesion molecules except Ng-CAM and neuroglian. Several possible sites for phosphorylation are present in the cytoplasmic tail of NILE. Antisera were produced against two NILE-beta-galactosidase fusion proteins containing distinct segments of the NILE polypeptide: the cytoplasmic domain and the segment containing fibronectin type III repeats. Immunoblots with these antisera and Northern blots with a NILE cDNA probe indicate that NILE continues to be expressed in most areas of the mature rat brain. This contradicts previous immunofluorescence data, which suggested that NILE was substantially down-regulated in maturing nerve fiber tracts. This raises the possibility that NILE could be masked in situ by interactions with other cell surface molecules.

No abstract