Abstract: This invention relates to a method of producing a circularised form of a target protein from a fusion protein and to a fusion protein capable of producing a circularised form of a target protein. The fusion protein can comprise: the target protein; at least one circularisation site adjacent the target protein; and an enzyme domain capable of interacting with the at least one circularisation site and circularising the target protein. The target protein can be a membrane scaffold protein or a cyclotide such as SFTI, Vc1.1, Kalata B1 or MCOTI-II.