Serum amyloid A is a retinol binding protein that transports retinol during bacterial infection.
Bottom Line: Serum amyloid A (SAA) proteins are strongly induced in the liver by systemic infection and in the intestine by bacterial colonization, but their exact functions remain unclear.Mouse and human SAAs bound retinol with nanomolar affinity, were associated with retinol in vivo, and limited the bacterial burden in tissues after acute infection.Our results thus identify SAAs as a family of microbe-inducible retinol binding proteins, reveal a unique protein architecture involved in retinol binding, and suggest how retinol is circulated during infection.
Affiliation: Department of Immunology, University of Texas Southwestern Medical Center, Dallas, United States.Show MeSH
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Mentions: SAAs lack sequence homology to the two known families of retinol binding proteins: cellular retinol binding proteins (CRBP) and serum retinol binding proteins (RBP) (Blaner, 1989; Noy, 2000). Thus, the three-dimensional structures of CRBP and RBP proteins (Newcomer et al., 1984; Cowan et al., 1993) provide no direct insight into the structural basis for retinol binding by SAAs. To understand how SAAs bind retinol, we therefore determined the three-dimensional structure of recombinant mSAA3 by X-ray crystallography. The protein was crystallized in a P62 space group with two subunits in the asymmetric unit, and the structure was determined to a resolution of 2 Å by single-wavelength anomalous dispersion (SAD) phasing using a selenomethionyl-derivatived crystal (Figure 4A; Table 2). The crystal structure reveals that mSAA3 is highly α-helical (Figure 4A), as predicted on the basis of its primary sequence (Figure 4—figure supplement 1) (Stevens, 2004). The structure is very similar to the recently determined structure of human SAA1.1 (Lu et al., 2014), an isoform that has a marked tendency to form pathogenic amyloid fibrils (Yu et al., 2000). Like the SAA1.1 structure, the mSAA3 structure consists of four α-helices, designated α1-4 from the N- to the C-termini, forming a cone-shaped four-helix bundle with a comparatively longer α1. The helices form two sets of antiparallel helices, α1-α2 and α3-α4, connected by a very short loop (Figure 4A). The monomer is stabilized by an extensive network of hydrogen bonding interactions among conserved residues and water molecules in the interior of the monomer. As in the SAA1.1 structure, the C-terminal tail wraps around the helix bundle making a number of hydrogen bonding interactions that add to monomer stability, underscoring the importance of the C-terminal tail.10.7554/eLife.03206.013Figure 4.Structure of mSAA3 and molecular contacts within the tetrameric unit.
Affiliation: Department of Immunology, University of Texas Southwestern Medical Center, Dallas, United States.