Involved in the biosynthesis of the mercapturic acid derivative diterpene cyslabdan A, a potentiator of the beta-lactam antibiotic imipenem (PubMed:26507838). Catalyzes the conversion of geranylgeranyl diphosphate (GGDP) into (+)-copalyl diphosphate (PubMed:26507838, PubMed:26814669). (2E,6E,10E)-geranylgeranyl diphosphate = (+)-copalyl diphosphate The Asp-Xaa-Asp-Asp-Thr-Ala (DXDDTA) and Gln-Xaa-Xaa-Asp-Gly-Ser-Trp (QXXDGSW) motifs are expected to bind to Mg(2+). Belongs to the terpene synthase family.