Serum Amyloid A is a Retinol Binding Protein that Transports Retinol during Bacterial Infection
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Retinol plays a vital role in the immune response to infection, however it remains unclear which proteins mediate retinol transport during infection. Serum amyloid A (SAA) proteins are produced by the liver following acute systemic infection and are also induced by bacteria in the intestine. SAAs have been proposed to play a role in the inflammatory response to infection and injury, but their exact functions have not been well defined. In this dissertation, I present data that demonstrates the acute phase protein SAA is a novel retinol binding protein that transports retinol during infection. SAA proteins are induced by bacteria and additionally require retinol for their expression. I demonstrate that SAA’s requirement for retinol is not restricted to the small intestine, as mice on a vitamin A deficient diet have reduced SAA expression in the liver as well. Additionally, I demonstrate in fluorescence based binding assays that SAAs are capable of binding retinol at nanomolar affinities, which is comparable to a known retinol binding protein. I also found that SAA proteins associate with retinol in the serum following a bacterial challenge in wild-type mice. This phenotype was not observed in SAA1/2-/- mice following bacterial challenge. Furthermore, SAA1/2-/- mice have greater bacterial loads in their spleens and livers following bacterial infection. In parallel with my studies, Dr. Mehabaw Derebe, a post-doctoral researcher in the Hooper lab, recently solved the mSAA3 crystal structure, demonstrating the protein oligomerizes to form a tetramer. This tetramer unit contains a central pore-like cavity, lined with hydrophobic amino acid residues, which would allow a lipophilic ligand to bind. A single amino acid mutation within this hydrophobic core resulted in reduced mSAA3 retinol binding. This structural insight describes how SAA, as a small and mostly alpha-helical protein, can protect a lipophilic ligand from the aqueous environment. Altogether, these data demonstrated that SAAs are a family of microbe-induced retinol binding proteins, reveal a unique protein architecture involved in retinol binding, and provide insight into the acute response to infection.