Performs the first committed step in the biosynthesis of isoprene-containing compounds such as sterols and terpenoids (By similarity). Component of the triterpenes (e.g. ginsenosides or panaxosides) and phytosterols biosynthetic pathways (PubMed:24933610, PubMed:29378087). Promotes the accumulation of stigmasterol and beta-sitosterol (PubMed:24933610). (R)-5-diphosphomevalonate + ATP = ADP + CO2 + isopentenyl diphosphate + phosphate Isoprenoid biosynthesis; isopentenyl diphosphate biosynthesis via mevalonate pathway; isopentenyl diphosphate from (R)-mevalonate: step 3/3. Homodimer. Belongs to the diphosphomevalonate decarboxylase family.