Bacteriorhodopsin precursor. Characterization and its integration into the purple membrane.

Halobacterium halobium spheroplasts synthesize and accumulate a bacteriorhodopsin precursor. By labeling of the precursor with [35S]Met and [3H]Leu followed by Edman degradation, we have confirmed the previous conclusion from the DNA sequencing that the precursor contains 13 additional amino acids at the NH2 terminus of bacteriorhodopsin. Although not processed in the spheroplasts, it integrates into the purple membrane in the correctly folded conformation. This was shown by the mode of cleavage by a number of proteolytic enzymes, the site of attachment of retinal, and the formation of oligomers on reaction with bifunctional cross-linking reagents. In all these respects, the behavior of the precursor was identical with that of native mature bacteriorhodopsin in the purple membrane. Finally, the precursor was not processed to bacteriorhodopsin even when the spheroplasts were subsequently allowed to revert to rod-shaped cells. This suggests that either the processing of the precursor is cotranslational or that the NH2 terminus of the precursor becomes inaccessible to the processing enzyme in the spheroplasts following integration into the membrane.