THE ARCHAEBACTERIAL MEMBRANE-PROTEIN BACTERIOOPSIN IS EXPRESSED AND N-TERMINALLY PROCESSED IN THE YEAST SACCHAROMYCES-CEREVISIAE

The bop gene codes for the membrane protein bacterio-opsin (BO), which on binding all-trans-retinal, constitutes the light-driven proton pum p bacteriorhodopsin (BR) in the archaebacterium Halobacterium salinari um(3). This gene was cloned in a yeast multi-copy vector and expressed in Saccharomyces cerevisiae under the control of the constitutive ADH 1 promoter. Both the authentic gene and a modified form lacking the pr ecursor sequence were expressed in yeast. Both proteins are incorporat ed into the membrane in S. cerevisiae. The presequence is thus not req uired for membrane targeting and insertion of the archaebacterial prot ein in budding yeast, or in the fission yeast Schizosaccharomyces pomb e, as has been shown previously. However, in contrast to S. pombe tran sformants, which take on a reddish colour when all-trans-retinal is ad ded to the culture medium as a result of the in vivo regeneration of t he pigment, S. cerevisiae cells expressing BO do not take on a red col our. The precursor of BO is processed to a protein identical in size t o the mature BO found in the purple membrane of Halobacterium. The eff iciency of processing in S. cerevisiae is dependent on growth phase, a s well as on the composition of the medium and on the strain used. The efficiency of processing of BR is reduced in S. pombe and in a retina l-deficient strain of H. salinarium, when retinal is present in the me dium.