Involved in the biosynthesis of the lanaridin cypemycin. [cypemycin](1-18)-L-Cys-L-Leu-L-Val-L-Cys + A = AH2 + C(3,19),S(21)-[cypemycin](1-18)-L-Ala-L-Leu-N-thioethenyl-L-valinamide + CO2 + hydrogen sulfide Belongs to the HFCD (homooligomeric flavin containing Cys decarboxylase) superfamily.