With NUS1, forms the dehydrodolichyl diphosphate synthase (DDS) complex, an essential component of the dolichol monophosphate (Dol-P) biosynthetic machinery. Both subunits contribute to enzymatic activity, i.e. condensation of multiple copies of isopentenyl pyrophosphate (IPP) to farnesyl pyrophosphate (FPP) to produce dehydrodolichyl diphosphate (Dedol-PP), a precursor of dolichol phosphate which is utilized as a sugar carrier in protein glycosylation in the endoplasmic reticulum (ER). Synthesizes long-chain polyprenols, mostly of C95 and C100 chain length. Regulates the glycosylation and stability of nascent NPC2, thereby promoting trafficking of LDL-derived cholesterol. (2E,6E)-farnesyl diphosphate + n isopentenyl diphosphate = di-trans,poly-cis-polyprenyl diphosphate + n diphosphate Binds 1 magnesium ion per subunit. Protein modification; protein glycosylation. Lipid metabolism. Forms an active dehydrodolichyl diphosphate synthase complex with NUS1. Interacts with NPC2. colocalizes with calnexin. The catalytic site at NUS1-DHDDS interface accomodates both the allylic and the homoallylic IPP substrates to the S1 and S2 pockets respectively. The beta-phosphate groups of IPP substrates form hydrogen bonds with the RXG motif of NUS1 and four conserved residues of DHDDS (Arg-85, Arg-205, Arg-211 and Ser-213), while the allylic isopentenyl group is pointed toward the hydrophobic tunnel of the S1 pocket where the product elongation occurs. Belongs to the UPP synthase family.