Glycobiology of surface layer proteins

Over the last two decades, a significant change of perception has taken pla ce regarding prokaryotic glycoproteins. For many years, protein glycosylati on was assumed to be limited to eukaryotes; but now, a wealth of informatio n on structure, function, biosynthesis and molecular biology of prokaryotic glycoproteins has accumulated, with surface layer (S-layer) glycoproteins being one of the best studied examples. With the designation of Archaea as a second prokaryotic domain of life, the occurrence of glycosylated S-layer proteins had been considered a taxonomic criterion for differentiation bet ween Bacteria and Archaea. Extensive structural investigations, however, ha ve demonstrated that S-layer glycoproteins are present in both domains. Amo ng Gram-positive bacteria, S-layer glycoproteins have been identified only in bacilli. In Gram-negative organisms, their presence is stili not fully i nvestigated; presently, there is no indication for their existence in this class of bacteria. Extensive biochemical studies of the S-layer glycoprotei n from Halobacterium halobium have, at least in part, unravelled the glycos ylation pathway in Archaea; molecular biological analyses of these pathways have not been performed, so far. Significant observations concern the occu rrence of unusual linkage regions both in archaeal and bacterial S-layer gl ycoproteins. Regarding S-layer glycoproteins of bacteria, first genetic dat a have shed some light into the molecular organization of the glycosylation machinery in this domain. In addition to basic S-layer glycoprotein resear ch, the biotechnological application potential of these molecules has been explored. With the development of staightforward molecular biological metho ds, fascinating possibilities for the expression of prokaryotic glycoprotei ns will become available. S-layer glycoprotein research has opened up oppor tunities for the production of recombinant glycosylation enzymes and tailor -made S-layer glycoproteins in large quantities, which are commercially not yet available. These bacterial systems may provide economic technologies f or the production of biotechnologically and medically important glycan stru ctures in the future. (C) 2001 Societe francaise de biochimie et biologie m oleculaire / Editions scientifiques et medicales Elsevier SAS. All rights r eserved.