Involvement of a 40-kDa Glycoprotein in Food Recognition, Prey Capture, and Induction of Phagocytosis in the Protozoon Actinophrys sol

Abstract

A 40-kDa glycoprotein (gp40) was identified as a Con A-binding adhesive substance of the heliozoon Actinophrys sol for immobilizing and ingesting prey flagellates. Isolation and partial characterization of gp40 showed that: 1) gp40 is a major Con A-binding protein of Actinophrys with a molecular weight of 40 kDa, and is stored in secretory granules called extrusomes; 2) gp40 was purified by Con A-affinity chromatography, and the N-terminal amino acid sequence was determined as H2N-KVLK-FEDDFDTFDLQ; 3) prey flagellates became adhered to gp40-immobilized agarose beads; 4) phagocytosis of Actinophrys was induced against gp40-immobilized agarose beads; and 5) solubilized gp40 induced exocytosis of extrusomes and cell fusion of heliozoons. These results indicate that gp40 is a multi-functional secretory protein of Actinophrys, which is required in correct targeting of the heliozoon to food organisms as well as in self-recognition.