A membrane-bound adhesive protein that promotes neurite outgrowth in brain neurons has been isolated from rat brain (Rauvala, H., and R. Pihlaskari. 1987. J. Biol. Chem. 262:16625-16635). The protein is an immunochemically distinct molecule with a subunit size of approximately 30 kD (p30). p30 is an abundant protein in perinatal rat brain, but its content decreases rapidly after birth. In the present study the amino-terminal sequence of p30 was determined by automated Edman degradations. A single amino-terminal sequence was found, which is not present in previously studied adhesive molecules. This unique sequence has a cluster of five positive charges within the first 11 amino acid residues: Gly-Lys-Gly-Asp-Pro-Lys-Lys-Pro-Arg-Gly-Lys. Antisynthetic peptide antibodies that recognize this sequence were produced in a rabbit, purified with a peptide affinity column, and shown to bind specifically to p30. The antipeptide antibodies were used, together with anti-p30 antibodies, to study the localization of p30 in brain cells and in neuroblastoma cells as follows. (a) Immunofluorescence and immunoelectron microscopy indicated that p30 is a component of neurons in mixed cultures of brain cells. The neurons and the neuroblastoma cells expressed p30 at their surface in the cell bodies and the neurites. In the neurites p30 was found especially in the adhesive distal tips of the processes. In addition the protein was detected in ribosomal particles and in intracellular membranes in a proportion of cells. (b) The antibodies immobilized on microtiter wells enhanced adhesion and neurite growth indicating that p30 is surface exposed in adhering neural cells. (c) Immunoblotting showed that p30 is extracted from suspended cells by heparin suggesting that a heparin-like structure is required for the binding of p30 to the neuronal cell surface. A model summarizing the suggested interactions of p30 in cell adhesion and neurite growth is presented.

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