To clone and characterize the squalene synthase from Siraitia grosvenorii (SgSQS). The gene encoding SgSQS was cloned. SgSQS has 417 amino acid residues with an pI of 7.3. There are 32 phosphorylation sites in its sequence: S-48 as well as S-196 play important roles in regulation of enzyme activity. The enzyme is a monomeric protein with a cave-like active center formed by alpha helixes and has two transmembrane domains at its C-terminus. SgSQS mRNA expression in stem and root were about twice as much as that in leaf and peel. Full-length SgSQS with measurable catalytic activity was expressed in Escherichia coli. SgSQS activity was optimal at 37 A degrees C and pH 7.5 respectively. SgSQS gene was cloned, and the molecular structure and biochemical function of SgSQS were characterized.