Dengue fever has re-emerged as a global health risk over the past 10 years. The Dengue virus is transmitted by the Aedes aegypti mosquito, which becomes infected with the virus upon blood feeding on an infected human host. Blood feeding, and hence blood meal digestion, is required to obtain the proper nutrients for completion of the gonotrophic cycle. Serine proteolytic enzymes digest the blood meal proteins, and although there are functional studies that have demonstrated the important roles these serine proteases play in blood meal metabolism, no one has biochemically characterized these proteases in vitro. I have engineered recombinant protease constructs that enable us to express, purify, and activate mosquito proteases in vitro using a bacterial expression system and a denaturation/refolding strategy. The four major midgut proteases studied (AaET, AaLT, AaSPVI, and AaSPVII), were purified to near homogeneity and were shown to be active in in vitro enzyme assays. Kinetic parameters, using the artificial trypsin substrate BApNA, were determined for three of the four mosquito proteases. AaLT, which was originally believed to be the major trypsin involved in blood meal digestion in the later phase portion of digestion, was shown not to be a classic trypsin based on in vitro BApNA assays. However, this same AaLT enzyme preparation was shown to cleave natural blood meal protein substrates (serum albumin and hemoglobin), confirming its proteolytic activity. Determination of mosquito protease activities and partial proteolysis of bovine serum albumin (BSA) provide evidence that the four proteases have selective cleavage sites. We also present initial evidence that mosquito proteases may be autocatalytic, based on in vitro auto-cleavage assays with AaET and AaLT.

Transition metals such as copper, zinc and nickel are required in many enzymatic processes that require redox changes. When transition metal concentration exceeds a certain threshold, their redox and metal binding properties make these elements extremely toxic. Bacteria regulate the cellular concentration of these important, yet toxic, elements using elaborate homeostatic systems. One such mechanism is the chemiosmotic extrusion of copper by the Cus system in the Gram-negative bacterium Escherichia coli. This work studies the regulation of the Cus system in response to copper and silver ions. Copper is an essential cofactor required in many enzymatic processes. But its redox properties can lead to toxicity. Silver is chemically similar to copper, but is not bioactive and its presence in cells can lead to extreme cytotoxicity. Transcription from cusCFBA genes is controlled by the CusR/CusS TCS in response to elevated levels of copper or silver in the periplasmic space of E. coli. Extracellular signals are transduced into the cell through phosphotransfer reactions between the prototypical histidine kinase CusS and the response regulator CusR. Copper sensing by the periplasmic domain of CusS is proposed to initiate signal transduction in the Cus system. Despite the frequency with which bacteria employ histidine kinases to sense their environment, signal recognition and incorporation by the protein is not well understood. The goal of this research is to investigate the role of CusS in regulating metal homeostasis in E. coli and characterize the periplasmic domain of the protein to determine its metal binding properties. The experiments described in this work reveal that the CusS is essential for copper and silver resistance and regulates expression from the cusCFBA promoter region. Signal recognition occurs by direct metal binding by the periplasmic domain of CusS. Metal binding causes a change in the secondary structure of the domain and its tendency to dimerize is enhanced under these conditions. The possibility of signal attenuation by interaction with the metallochaperone CusF is also discussed. These data help construct a model for signal transduction in the Cus system and help characterize, for the first time, a metal-responsive sensor histidine kinase in E. coli.