Abstract: In insects, cholesterol is not only one of the main components of cell membranes, but also a precursor of ecdysone biosynthesis. However, because insects lack two key enzymes for cholesterol biosynthesis, they can not autonomously synthesize cholesterol from simple compounds and therefore have to obtain sterols from their diet. Insects must convert food sterols into cholesterol to meet the requirements of growth, development and reproduction. Sterol carrier proteins (SCPs) are main transport proteins for sterol absorption and transport in insects. It is critical to study the relationship between structure and function of SCPs for understanding the roles of SCPs in sterol transport. In this review, recent progress in the study of the structure, expression and distribution of SCP genes and proteins, post-translation modification, crystal structure, ligand-binding specificity and possible absorption and transport pathways of insect SCPs was summarized and the potential of using SCPs as a molecular target for pest insect control was also discussed. Studies indicate that transcript expression of SCP genes and post-translation modifications of SCP proteins vary depending on different species. In dipteran insects such as Aedes aegypti and Drosophila melangoster SCP-x gene encodes SCP-x and SCP-2 proteins, while there are additional SCP-2 genes and SCP-2-like genes encoding SCP-2 and SCP-2-like proteins, respectively. In lepidopteran insects such as Spodoptera littoralis, Spodoptera litura and Bombyx mori, the transcript expression and translation processes of SCP-x gene are similar to those in vertebrates, in which SCP-2 protein is produced after post-transcription and translation modifications of a unique SCP-x gene. SCP-x and SCP-2 proteins are localized in peroxisomes. SCP-2 protein consists of 5 αhelixes and 5 β-sheets and the α5-helix appears to impact the binding of the protein to substrates. SCP-2 protein can bind with different affinity to cholesterol, cholesterol derivatives, fatty acids, acyl-coenzyme A and phospholipids. Over-expression of SlSCP-x and SlSCP-2 genes can increase the uptake of cholesterol into cells and RNAi inhibits the expression of SlSCP-x and SlSCP-2 genes in S. litura larvae, resulting in a decrease in cholesterol level in the hemolymph and a delay in larval growth and pupation.

Key words: Insect, sterol carrier protein, cholesterol, fatty acid, ecdysone