Computational characterization and in vivo expression of squalene synthase gene in different tissues of Artemisia annua L. plants

• Artemisinin is the key ingredient used in the form of ACTs to treat drug resistant malaria.
• SQS catalyzes the first step of sterol biosynthetic pathway that competes with artemisinin biosynthesis for FPP in A. annua L.
• The computational approaches of AaSQS might be helpful in modulating the triterpenoid biosynthesis in A. annua L. plants.

Artemisinin, a sesquiterpene lactone endoperoxide isolated from aerial parts of Artemisia annua L. plants is a safe and potent therapeutic agent against malaria causing drug-resistant parasite strains of Plasmodium sp. Squalene synthase (SQS) is a membrane-bound enzyme catalyzing the first enzymatic step of sterol biosynthetic pathway in eukaryotes. It competes with ADS, the rate limiting enzyme of artemisinin biosynthesis pathway in A. annua L. for the precursor FPP, the product of mevalonate/alternate pathway. Hence, SQS limits artemisinin biosynthesis in A. annua L. plants. In this study, full length cDNA fragment encoding AaSQS enzyme from the leaves of A. annua L. plants was cloned and analyzed. Computational characterization was performed for basic structural and functional information. The physico-chemical properties were also studied. The putative conserved domains in corresponding sequences were searched. The multiple sequence alignment of AaSQS revealed the important motifs and identified the residue substitution in each motif. The homology modeling of AaSQS enzyme was done for the prediction of 3D-structure. Molecular docking of one substrate and two co-factors with AaSQS was performed and the docked structures showed binding affinity for motif 2 of AaSQS. Our analysis further revealed that 29 residues of motif 2 might be important for catalytic/functional activity of AaSQS enzyme. Relative ontogenetic expression analysis of SqS gene employing quantitative real-time PCR showed that the expression level of AaSqS was higher in Callus (C) followed by leaves (L), stem (S) and hairy roots (HR) that was well correlated with the squalene content in these tissues. This study thus, may provide a platform to understand and use the predicted information of AaSQS in modulating the production of artemisinin in A. annua L. plants.