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Vsp surface lipoproteins are serotype-defining antigens of relapsing fever spirochetes that undergo multiphasic antigenic variation to avoid the immune response. One of these proteins, VspA of Borrelia turicatae, is also associated with neurotropism in infected mice. Vsp proteins are highly polymorphic in sequence, which may relate to their specific antibody reactivities and host cell interactions. To determine whether sequence variations affect protein structure, we compared B. turicatae VspA with three related proteins: VspB of B. turicatae, Vsp26 of the relapsing fever agent Borrelia hermsii, and OspC of the Lyme disease spirochete Borrelia burgdorferi. Recombinant non-lipidated proteins were purified by affinity or ion exchange chromatography. Circular dichroism spectra revealed similar, highly alpha-helical secondary structures for all four proteins. In vitro assays demonstrated protease-resistant, thermostable Vsp cores starting at a conserved serine at position 34 (Ser(34)). All proteins aggregate as dimers in solution. In situ trypsin treatment and surface protein cross-linking showed that the native lipoproteins also form protease-resistant dimers. These findings indicate that Vsp proteins have a common compact fold and that their established functions are based on localized polymorphisms. Two forms of VspA crystals suitable for structure determination by x-ray diffraction methods have been obtained.

Several lines of evidence suggest that the morphogenetic transition from the yeast form to pseudohyphae in Saccharomyces cerevisiae may be regulated by the cyclin-dependent kinase (Cdk). To examine this hypothesis, we mutated all of the G1 cyclin genes in strains competent to form pseudohyphae. Interestingly, mutation of each G1 cyclin results in a different filamentation phenotype, varying from a significant defect in cln1/cln1 strains to enhancement of filament production in cln3/cln3 strains. cln1 cln2 double mutants are more defective in pseudohyphal development and haploid invasive growth than cln1 strains. FLO11 transcription, which correlates with the level of invasive growth, is low in cln1 cln2 mutants and high in grr1 cells (defective in proteolysis of Cln1,2), suggesting that Cln1,2/Cdks regulate the pseudohyphal transcriptional program. Epistasis analysis reveals that Cln1,2/Cdk and the filamentation MAP kinase pathway function in parallel in regulating filamentous and invasive growth. Cln1 and Cln2, but not Ste20 or Ste12, are responsible for most of the elevated FLO11 transcription in grr1 strains. Furthermore, phenotypic comparison of various filamentation mutants illustrates that cell elongation and invasion/cell-cell adhesion during filamentation are separable processes controlled by the pseudohyphal transcriptional program. Potential targets for G1 cyclin/Cdks during filamentous growth are discussed.

In mice infected with serotype A but not serotype B of the relapsing fever spirochete Borrelia turicatae, early invasion of the brain occurs. Serotypes A and B are further distinguished by the abundant surface protein they produce: VmpA and VmpB, respectively. Western blotting with monoclonal antibodies, one-dimensional peptide mapping, and partial amino acid sequencing demonstrated regions of the VmpA protein that differed from VmpB. Oligonucleotide primers based on the partial amino acid sequences of unique regions were used to amplify a portion of the VmpA gene (vmpA) by PCR, and the product was used as a probe in Southern blot and Northern blot analyses. These experiments showed that (i) expression of the vmpA sequence was determined at the level of transcription and (ii) the vmpA sequence was in two locations in serotype A and one location in serotype B. The vmpA gene at the expression-linked locus of serotype A was cloned and sequenced. An open reading frame would encode a polypeptide of 214 amino acids. The polypeptide expressed by Escherichia coli was bound by VmA-specific but not VmpB-specific antibody. Primer extension analysis identified a consensus sigma70-type promoter for vmpA at the expression locus. Phylogenetic analysis revealed that VmpA is homologous to small Vmp (Vsp) proteins of B. hermsii and to OspC proteins of B. burgdorferi. These findings indicate that a function of the Vsp-OspC family of proteins of Borrelia spp. may be differential localization in organs, including the brain, during infection.