Surface
lipoproteins of Borrelia spirochetes are important virulence determinants in the transmission and pathogenesis of
Lyme disease and
relapsing fever. To further define the conformational secretion requirements and to identify potential
lipoprotein translocation intermediates associated with the bacterial outer membrane (OM), we generated constructs in which Borrelia burgdorferi outer surface
lipoprotein A (OspA) was fused to
calmodulin (CaM), a conserved eukaryotic
protein undergoing
calcium-dependent folding.
Protein localization assays showed that constructs in which CaM was fused to full-length wild-type (wt) OspA or to an intact OspA N-terminal "tether"
peptide retained their competence for OM translocation even in the presence of
calcium. In contrast, constructs in which CaM was fused to truncated or mutant OspA N-terminal tether
peptides were targeted to the periplasmic leaflet of the OM in the presence of
calcium but could be flipped to the bacterial surface upon
calcium chelation. This indicated that in the absence of an intact tether
peptide, unfolding of the CaM moiety was required in order to facilitate OM traversal. Together, these data further support a periplasmic tether
peptide-mediated mechanism to prevent premature folding of B. burgdorferi surface
lipoproteins. The specific shift in the OM topology of sequence-identical
lipopeptides due to a single-variable change in environmental conditions also indicates that surface-bound Borrelia
lipoproteins can localize transiently to the periplasmic leaflet of the OM.