Specific secretion of proline-rich proteins by salt-adapted winged bean cells

Six proteins, designated SAP1 through SAP6, were secreted specifically by salt-adapted cells of winged bean (Psophocarpus tetragonolobus) in suspension cultures. The amino-terminal amino acid sequences of SAP2 (57 kDa), SAP4 (21 kDa), SAP5 (19 kDa) and SAP6 (17 kDa) were homologous to the sequences of proline-rich proteins, indicating that proline-rich proteins are secreted specifically by these salt-adapted cells. In addition, the amino-terminal amino acid sequence of SAP2 was identical to that of SAP4, and the amino-terminal sequence of SAP5 was identical to that of SAP6. Secretion of SAP2 was significantly enhanced by addition of AlCl3 but not of KCl, LiCl, CaCl2, MgCl2, mannitol or sucrose to suspension cultures. Furthermore, secretion of SAP4, SAP5 and SAP6 was stimulated by addition of abscisic acid to cultures, suggesting that these proteins might be secreted in response to salt or osmotic stress.