Converts squalene to mono- and dimethyl derivatives, but not to tri- and tetramethylated products. Unable to methylate cycloartenol, zymosterol or lanosterol. Methylates both C-3 and C22 positions, but only C-3 position in monomethylated products. Produces mainly monomethylated squalene and only 20% of dimethylated squalene. 2 S-adenosyl-L-methionine + squalene = 3,22-dimethyl-1,2,23,24-tetradehydro-2,3,22,23-tetrahydrosqualene + 2 H(+) + 2 S-adenosyl-L-homocysteine Belongs to the class I-like SAM-binding methyltransferase superfamily. Erg6/SMT family.