Methyltransferase required for the conversion of 2-polyprenyl-6-methoxy-1,4-benzoquinol (DDMQH2) to 2-polyprenyl-3-methyl-6-methoxy-1,4-benzoquinol (DMQH2). a 2-methoxy-6-all-trans-polyprenyl-1,4-benzoquinol + S-adenosyl-L-methionine = a 6-methoxy-3-methyl-2-all-trans-polyprenyl-1,4-benzoquinol + H(+) + S-adenosyl-L-homocysteine Cofactor biosynthesis; ubiquinone biosynthesis. Component of a multi-subunit COQ enzyme complex, composed of at least COQ3, COQ4, COQ5, COQ6, COQ7 and COQ9. Widely expressed, with highest levels in liver, lung, placenta and skeletal muscle. The disease may be caused by variants affecting the gene represented in this entry. Belongs to the class I-like SAM-binding methyltransferase superfamily. MenG/UbiE family. Non-canonical splice intron-exon junction. Truncated N-terminus.