Identification of a bacterio‐opsin species with a N‐terminally extended amino acid sequence

Halobacterium halobium, when induced for purple membrane production, synthesizes the integral membrane protein bacteria-opsin at a much higher rate than other membrane proteins [ 11. Bacteria-opsin is therefore an attractive model for studying the problem of synthesis of intrinsic membrane proteins [2]. Protein synthesis of bacteria-opsin and some other membrane proteins in vivo is selectively disturbed when Mg2+ are removed from the medium, whereas no effect on the synthesis of cytoplasmic proteins can be observed. Re-addition of Mg2+ to the cell suspension reconstitutes an almost normal membrane protein pattern. Instead of bacterio-opsin however, a protein species with a slightly higher apparent molecular weight appears, which is referred to as 27 000 Mr protein [3]. Here we show that the 27 000 M, protein is a bacteria-opsin molecule which is elongated by an additional peptide at the N-terminus. The N-terminal amino acid sequence of the protein was determined up to position 5 by manual Edman radiosequencing. We found the sequence: