The sterol carrier protein-2 like 3 gene (AeSCP-2L3), a new member of the SCP-2 protein family, is identified from the yellow fever mosquito, Aedes aegypti. The predicted molecular weight of AeSCP-2L3 is 13.4 kDa with a calculated pI of 4.98. AeSCP-2L3 transcription occurs in the larval feeding stages and the mRNA levels decrease in pupae and adults. The highest levels of AeSCP-2L3 gene expression are found in the body wall, and possibly originated in the fat body. This is the first report of a mosquito SCP-2-like protein with prominent expression in tissue other than the midgut. The X-ray protein crystal structure of AeSCP-2L3 reveals a bound C16 fatty acid whose acyl tail penetrates deeply into a hydrophobic cavity. Interestingly, the ligand-binding cavity is slightly larger than previously described for AeSCP-2 (Dyer et al. J Biol Chem 278:39085–39091, 2003) and AeSCP-2L2 (Dyer et al. J Lipid Res M700460–JLR200, 2007). There are also an additional 10 amino acids in SCP-2L3 that are not present in other characterized mosquito SCP-2s forming an extended loop between β3 and β4. Otherwise, the protein backbone is exceedingly similar to other SCP-2 and SCP-2-like proteins. In contrast to this observed high structural homology of members in the mosquito SCP2 family, the amino acid sequence identity between the members is less than 30%. The results from structural analysis imply that there have been evolutionary constraints that favor the SCP-2 Cα backbone fold while the specificity of ligand binding can be altered.
Sterol carrier Mosquito Cholesterol Fatty acid
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This work was supported by the University of Wisconsin-Madison College of Agriculture and Life Sciences’ USDA-CSREES Hatch project WIS04963, by grant W9113 M-05-1-0006 from the Deployed War Fighter Protection Research Program (DWFP) administered by the US Armed Forces Pest Management Board (AFPMB), by the National Institute of Health research grant #5R01AI067422 to Q.L. and by the Alex and Lillian Feir Graduate Fellowship to I.V.
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