Cranin interacts specifically with the sulfatide-binding domain of laminin

Cranin is a 120 kDa integral membrane glycoprotein which binds laminin under conditions of physiologic ionic strength in a calcium-dependent manner. Here, binding of cranin to laminin has been characterized using both ligand-blotting assays and laminin affinity bead assays. Binding was specifically inhibited by anti-laminin antibodies against the A chain terminal domain G, but not by several other region-specific antibodies. Dextran sulfate, fucoidin, and sulfatide were potent inhibitors of binding (50% inhibition at 0.03, 0.5, and 1.7 micrograms/ml, respectively); heparin was a weaker inhibitor (50% inhibition approximately 5 micrograms/ml), and mannan and chondroitin sulfate were not inhibitory at 100 micrograms/ml. Binding was not inhibited by lactose or the A chain peptide PA22-2. The mobility of the broad, fuzzy cranin band was shifted after digestion with neuraminidase, N-glycanase, and O-glycanase, though none of these treatments decreased band heterogeneity nor destroyed the ability to bind laminin. Cranin bound to Jacalin lectin, which recognizes the Gal beta 1-3GalNAc linkage expressed in certain classes of mucins. These findings indicate that cranin binds at or near the high affinity sulfatide-binding site previously mapped to the E3 domain of laminin, which is known to exhibit bioactivity for neural cells. In view of the extremely low abundance of cranin in brain membranes (approximately 0.005%), its avid laminin-binding activity is remarkable, and strongly suggests that cranin may play a physiologic role in regulating specific neural cell interactions.