Mary Lou Ernst-Fonberg, M.D., Ph.D.

Professor Emeritus on half-time appointment 

Research Interests

Purposeful macromolecular association involves the interplay of conserved and nonconserved structural elements, which together define the specificity of the interaction. Protein-protein complex interfaces are relatively undistinguished surfaces with little significant or regular topology. It is a challenge to discover how a protein might distinguish the appropriate binding partner from many alternatives. Such discrimination lies at the heart of fine-tuning in biological processes such as signal transduction, hormone-receptor interactions, metabolic control, and, where proteins are substrates, enzyme-substrate complexes.
Acyl carrier protein (ACP) is a remarkably interactive protein in the course of its myriad metabolic roles which requires its specific recognition of many different proteins. How ACP recognizes and binds different proteins is not known. Initially, appreciation of ACP's interaction with diverse proteins came from its function as a substrate carrier in fatty acid and subsequent lipid biosynthesis in plants and bacteria. During fatty acid biosynthesis in chloroplasts and bacteria, ACP carries the common substrate to six separate enzymes for sequential processing. At the end of the process a long chain fatty acyl protein, acyl-ACP, emerges with some properties different from those of ACP. Conformational changes occur in conjunction with ACP's acylation. Indeed, it was the first of the internally fatty acylated proteins; a feature now appreciated to be a means of significantly changing a protein's behavior. The reversibility of fatty acylation of proteins acylated on internal amino acid residues provides an exquisite fine-tuning mechanism of turning on and off protein-protein interaction presumably by conformational alteration, which masks and unmasks recognition sites.
A rather dramatic demonstration of the effects of acylation on protein behavior is seen with the RTX (repeats in toxin) toxins secreted by various pathogeneic Gram negative bacteria. These toxins, typified by Escherichia coli hemolysin (HlyA) are proteins, and they are not toxic until they are fatty acylated on two internal lysine residues by a cosynthesized enzyme, HlyC, an internal protein fatty acyltransferase. Acyl-ACP is the obligatory donor of the fatty acyl group. Following acylation, the toxin can create lytic pores in mammalian target cell membranes including the immune system or at extremely low levels stimulate leukotriene production and disrupt cellular function. We have separately subcloned, expressed, and purified each of the proteins involved in activation of the toxin. We are defining the biochemistry of the internal acylation of a protein, and documenting the effects of acyl group chain length and unsaturation on the acylation reaction and on the toxicity of the resultant toxin. We are studying the biochemical basis of the specificity for acyl-ACP in the activation of hemolysin toxin and determining how the proteins recognize one another.
A diversity of techniques are used in these studies ranging from molecular biology techniques including site-directed mutagenesis to enzymolgy, chemical modification, and spectroscopic methods

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