Amphoterin, the 30-kDa protein in a family of HMG1-type polypeptides. Enhanced expression in transformed cells, leading edge localization, and interactions with plasminogen activation

Amphoterin is a heparin-binding protein that is developmentally regulated in brain and functionally involved in neurite outgrowth. Unexpectedly, amphoterin has a high mobility group 1 (HMG1)-type sequence. In the present study we have expressed amphoterin cDNA in a baculovirus vector and produced antibodies against the recombinant protein and several synthetic peptides. It was found that the amphoterin cDNA encodes the 30-kDa form of the protein isolated from tissues, whereas the co-purifying 28- and 29-kDa proteins (p28 and p29) have closely related but distinct primary structures. Partial amino acid sequencing shows several local changes in the sequences of p28 and p29 compared with amphoterin, suggesting the occurrence of a multigene family that encodes at least three different HMG1-type sequences in the rat. Studies using the probes that discern amphoterin from the other HMG1-type proteins indicate a high level expression in various transformed cell lines. Immunostaining of cells with the amphoterin-specific antibodies indicates a cytoplasmic localization that becomes remarkably enriched at the leading edges in spreading and motile cells. An extracellular localization is suggested by immunostaining of nonpermeabilized cells and by a plasminogen-dependent degradation of amphoterin in the substratum-attached material of cells. Tissue-derived and recombinant amphoterins strongly enhance the rate of plasminogen activation and promote the generation of surface-bound plasmin both by tissue-type and urokinase-type plasminogen activators. The results suggest an extracellular function for amphoterin in the leading edge of various invasive cells.