The prophenoloxidase activating system and associated proteins in invertebrates

In this review, we present arguments indicating that prophenoloxidase (proPO) activating system acts as a pattern recognition and defence system in invertebrate blood. Phenoloxidase (PO) activity has been found in the blood of many invertebrates. At least in arthropods, echinoderms and urochordates, the inactive pro-form, proPO has been found to be elicited by the microbial cell-wall components beta-1, 3-glucans, lipopolysaccharide and/or peptidoglycan. This activation seems to involve elicitor-binding proteins and serine protease(s). ProPO, the proPO-activating enzyme (ppA) and plasma elicitor-binding proteins, have been purified from some arthropods, and proPO and the beta-1, 3-glucan binding protein (beta GBP) have been cloned and sequenced from crayfish. Arthropod proPO has a molecular mass of 70-90 kDa and PO has a molecular mass of 60-70 kDa. The beta GBP also stimulates phagocytosis of fungal cells and, after reacting with beta-1, 3-glucan, blood-cell degranulation (and release of the proPO system). In addition, a cell-adhesion protein (of 70-100 kDa), apparently associated with the proPO system, has been purified from arthropods. This mediates blood-cell adhesion, degranulation, phagocytosis and encapsulation. The cell-adhesion protein and beta GBP bind to a common blood-cell membrane receptor. It would be interesting to see the sequences of more proPO system components and investigate whether the scheme for cellular communication and defence, involving the cell-adhesion protein, elicitor-binding proteins and the membrane receptor described in arthropods, applies to invertebrates in general.