Post-translational modification of the S-layer glycoprotein occurs following translocation across the plasma membrane of the haloarchaeon Haloferax volcanii

The halophilic archaeon Haloferax volcanii is surrounded by a protein shell solely comprised of the S-layer glycoprotein. While the gene sequence and glycosylation pattern of the protein and indeed the three-dimensional struc ture of the surface layer formed by the protein have been described, little is known of the biosynthesis of the S-layer glycoprotein. In the following , pulse-chase radiolabeling and cell-fractionation studies were employed to reveal that newly synthesized S-layer glycoprotein undergoes a maturation step following translocation of the protein across the plasma membrane. The processing step, detected as an increase in the apparent molecular mass of the S-layer glycoprotein, is unaffected by inhibition of protein synthesis and is apparently unrelated to glycosylation of the protein. Maturation re quires the presence of magnesium ions, involved in membrane association of the S-layer glycoprotein, and results in increased hydrophobicity of the pr otein as revealed by enhanced detergent binding. Thus, along with protein g lycosylation, additional post-translational modifications apparently occur on the external face of the haloarchaeal plasma membrane, the proposed topo logical homologue of the lumenal face of the eukaryal endoplasmic reticulum membrane.