-!- Catalyzes the first pathway-specific step in the biosynthesis of the core membrane diether lipids in archaebacteria. -!- It catalyzes the alkylation of the primary hydroxy group in sn-glycerol 1-phosphate by geranylgeranyl diphosphate (GGPP) in a prenyltransfer reaction where a hydroxy group is the nucleophile in the acceptor substrate. -!- The other enzymes involved in the biosynthesis of polar lipids in archaea are EC 1.1.1.261, EC 2.5.1.42 and EC 2.7.7.67, which lead to the formation of CDP-unsaturated archaeol. -!- The final step in the pathway involves the addition of L-serine, with concomitant removal of CMP, leading to the production of unsaturated archaetidylserine.