Spirochaetal lipoproteins and pathogenesis.

Anchoring of proteins to membranes by lipid modification is a universal strategy of both prokaryotic and eukaryotic cells. The spirochaetes represent a distinct bacterial phylum that have utilized the lipoprotein design extensively as evidenced by genome-sequencing studies revealing a large number of different paralogous families of lipoprotein genes, encoding 22 potential lipoproteins in Treponema pallidum (Fraser et al., 1998) and 105 potential lipoproteins in Borrelia burgdorferi (Fraser et al., 1997). Roughly 8% of Bor. burgdorferi open reading frames are predicted to encode lipoproteins (Fraser et al., 1997), which is a significantly higher frequency than that of any other bacterial genome sequenced to date (The Institute for Genomic Research, 2000). An understanding of spirochaetal lipoproteins requires an appreciation of the distinctive doublemembrane architecture of spirochaetes, which shares characteristics of both Gram-positive and Gram-negative bacteria (see Fig. 1). As in Gram-positive bacteria, the cytoplasmic membrane of spirochaetes is closely associated with the peptidoglycan cell wall. Spirochaetes also have an outer membrane which provides a barrier shielding underlying antigens, such as the endoflagella, from the outside environment. However, the spirochaetal outer membrane appears to be fluid and labile, which contrasts it with the outer membrane of Gramnegative bacteria. This unique membrane architecture of spirochaetes and their ancient phylogeny suggest that the export, structure and function of spirochaetal lipoproteins have features which are unique to these organisms. As an indication of their importance in spirochaetes, lipoproteins are recognized to be the most abundant proteins in a wide array of organisms. Examples of lipoproteins that constitute a high percentage of the total protein composition of spirochaetes include OspA of Bor. burgdorferi, the Vmp proteins of the relapsing fever borreliae, SmpA of Brachyspira hyodysenteriae, LipL32 of the pathogenic Leptospira species and Tpp47 of T. pallidum. This paper is intended as a broad review of key issues involving spirochaetal lipoproteins, relating fundamental biochemical and physiological issues to those of pathogenesis. Rather than an exhaustive survey of current knowledge, the goal of this review is to highlight issues of current interest and point out areas for future research. Readers desiring more detailed information about spirochaetal lipoproteins are referred to additional sources, including websites such as the TIGR Microbial Database (The Institute for Genomic Research, 2000) and other reviews (e.g. Norris, 1993). The starting point for this review is lipoprotein secretion, processing and sorting, which will allow formulation of questions regarding how cellular destination is determined. With the exponential expansion of sequence data, what should be the criteria for identification of spirochaetal lipoproteins? Characterization of lipoproteins should also include determination of whether it is exported to the outer membrane and}or surface exposed, since surface-exposed lipoproteins are likely to be involved in how spirochaetes interact with their environment. Surface-exposed lipoproteins that have been implicated in essential roles in the interaction with the mammalian host will be reviewed. What are the environmental signals that regulate differential expression of lipoproteins? Some of the lipoproteins upregulated in the mammalian host undergo antigenic variation as a strategy for evading the host immune response. Lipoproteins are also involved in disease pathogenesis through their ability to trigger the host inflammatory response. Ultimately, lipoproteins are of interest as vaccines for prevention of spirochaetal infections.