Converts geranylgeranyl diphosphate to an new 5,7-fused bicyclic diterpene, named pseudolaratriene (PubMed:28096378). Catalyzes the first committed step in pseudolaric acid B (PAB) biosynthesis (PubMed:28096378). PAB exhibits antiproliferative activity by inhibiting microtubule polymerization, and has demonstrated antitumor properties against several cancer types (Probable). (2E,6E,10E)-geranylgeranyl diphosphate = diphosphate + pseudolaratriene Terpene metabolism. Expressed in young and mature roots (PubMed:28096378). Expressed at low levels in barks (PubMed:28096378). Belongs to the terpene synthase family.