Borrelia spirochaetes are unique among diderm bacteria in their abundance of 
surface-displayed lipoproteins, some of which play important roles in the 
pathogenesis of Lyme disease and relapsing fever. To identify the 
lipoprotein-sorting signals in Borrelia burgdorferi, we generated chimeras 
between the outer surface lipoprotein OspA, the periplasmic oligopeptide-binding 
lipoprotein OppAIV and mRFP1, a monomeric red fluorescent reporter protein. 
Localization of OspA and OppAIV point mutants showed that Borrelia lipoproteins 
do not follow the '+2' sorting rule which targets lipoproteins to the 
cytoplasmic or outer membrane of Gram-negative bacteria via the Lol pathway. 
Fusions of mRFP1 to short N-terminal lipopeptides of OspA, and surprisingly 
OppAIV, were targeted to the spirochaetal surface. Mutagenesis of the OspA 
N-terminus defined less than five N-terminal amino acids as the minimal 
secretion-facilitating signal. With the exception of negative charges, which can 
act as partial subsurface retention signals in certain peptide contexts, 
lipoprotein secretion occurs independent of N-terminal sequence. Together, these 
data indicate that Borrelia lipoproteins are targeted to the bacterial surface 
by default, but can be retained in the periplasm by sequence-specific signals.