PLASMINOGEN BINDS SPECIFICALLY TO ALPHA-ENOLASE ON RAT NEURONAL PLASMA-MEMBRANE

Plasminogen (PGn) that we identified in microglial-conditioned medium has a neurotrophic factor-like effect on cultured neurons. We have als o shown that PGn binds specifically to a protein with a molecular mass of 45 kDa in the neuronal plasma membrane. As a candidate PGn recepto r-like molecule on the neuronal surface, this 45-kDa protein was purif ied from the plasma membrane of embryonic rat brain. Amino acid sequen ce analysis of polypeptides derived from the cleavage of the protein w ith cyanogen bromide and V8 protease revealed that the 45-kDa protein is identical to rat alpha-enolase. In fact, PGn was found to bind to p urified rat alpha-enolase and also to a synthetic peptide (30 residues ) that corresponds to the carboxyl terminal region of rat alpha-enolas e. Physical properties of the 45-kDa protein, such as molecular mass, isoelectric point, and the ability to form dimers, are quite similar t o those of alpha-enolase. The 45-kDa PGn-binding protein in the plasma membrane was also recognized by anti-rat alpha-enolase antibody, and pretreatment with alpha-enolase antibody markedly diminished the PGn-b inding to the plasma membrane. In addition, immunocytochemical stainin g of the cultured cells under the nonpermeable condition showed that a lpha-enolase is present on the cell surface of a certain population of neurons. These results suggest that alpha-enolase may function as a P Gn-binding molecule on the neuronal cell surface.