A family of surface-exposed proteins of 20 kilodaltons in the genus Borrelia

Abstract

Relapsing fever and Lyme disease spirochetes of the genus Borrelia display at their surfaces abundant lipoproteins: Vmp proteins in Borrelia hermsii and Osp proteins in Borrelia burgdorferi. Vmp and Osp proteins largely determine serotype specificity, and neutralizing antibodies of infected or immunized animals are directed at them. For the present study, we examined B. hermsii serotype 33, which is unique among strain HS1 serotypes in the low frequency of switches to other serotypes during infections and in vitro cultivation. Failing to clone the complete vmp33 gene, we accomplished its further characterization by (i) determining three partial amino acid sequences, (ii) designing oligonucleotide primers based on these amino acid sequences, (iii) cloning and sequencing the central portion of vmp33, and (iv) using outwardly directed primers and the inverse PCR to clone the 5' and 3' ends of the gene and flanking regions. The transcriptional start site was identified by primer extension analysis. Vmp33 was a polypeptide of 211 amino acids; the three partial amino acid sequences were identified in the open reading frame. Vmp33 was found to be more similar to other 20-kDa Vmp proteins of B. hermsii and to OspC proteins of B. burgdorferi than it was to 35- to 39-kDa Vmp proteins of the same strain. Moreover, OspC proteins were more similar to Vmp33 than they were to OspA, -B, or -D proteins of B. burgdorferi. These sequence similarities were consistent with Western blot (immunoblot) findings of cross-reactions between Vmp33 and OspC with anti-Vmp33 and anti-OspC sera. The promoter for the expressed vmp33 gene was found to be different from the expression site for other active vmp genes characterized to date. These results indicate that Vmp33 and other small Vmp's belong with OspC to a genus-wide family of 20-kDa proteins and that expression of these proteins may be coordinated with expression of other Vmp and Osp proteins in Borrelia spp.