Better knowledge of insect immune systems has also led in recent years to important advances in understanding human immunity. Nevertheless, the signaling systems used in recognising and responding to bacteria remain poorly understood. A protein, SPH-3, present in the hemolymph (blood) of a large experimental model insect, Manduca sexta, plays a key role in this insect's ability to defend itself against infection by pathogenic bacteria. SPH-3 is a member of the serine proteinase familiy of proteins, but is catalytically inactive. It appears to play a signalling role in the insect's immune system, but its precise role is unclear. The proposed work seeks to determine with which other immune system proteins SPH-3 interacts, and how it exerts its downstream effects. The work will lead to enhanced understanding of insect immune systems, and will thereby provide information that may also enable better understanding of immunity in man and other mammals. The work is of wide biological interest in that it concerns the biological functions of an enzyme that has evolved to gain a signaling function, but in doing so has lost its original catalytic activity. Further, since the protein in question is the target of a major virulence gene of a widely used agent of biological insect pest control, there may also be applied spin-off.