Treponema pallidum subsp. pallidum, the spirochete that causes
syphilis, is unusual in a number of respects, including its small genome size, inability to grow under standard in vitro culture conditions, microaerophilism, apparent paucity of outer
membrane proteins, structurally complex periplasmic flagella, and ability to evade the host immune responses and cause disease over a period of years to decades. Many of these attributes are related ultimately to its
protein content. Our knowledge of the activities, structure, and immunogenicity of its
proteins has been expanded by the application of
recombinant DNA, hybridoma, and structural fractionation techniques. The purpose of this monograph is to summarize and correlate this new information by using two-dimensional gel electrophoresis,
monoclonal antibody reactivity, sequence data, and other properties as the bases of
polypeptide identification. The
protein profiles of the T. pallidum subspecies causing
syphilis,
yaws, and endemic
syphilis are virtually indistinguishable but differ considerably from those of other treponemal species. Among the most abundant
polypeptides are a group of
lipoproteins of unknown function that appear to be important in the immune response during syphilitic
infection. The periplasmic flagella of T. pallidum and other spirochetes are unique with regard to their
protein content and ultrastructure, as well as their periplasmic location. They are composed of three core
proteins (homologous to the other members of the eubacterial
flagellin family) and a single, unrelated sheath
protein; the functional significance of this arrangement is not understood at present. Although the bacterium contains the
chaperonins GroEL and DnaK, these
proteins are not under the control of the heat shock regulon as they are in most organisms. Studies of the immunogenicity of T. pallidum
proteins indicate that many may be useful for immunodiagnosis and immunoprotection. Future goals in T. pallidum
polypeptide research include continued elucidation of their structural locations and functional activities, identification and characterization of the low-abundance outer
membrane proteins, further study of the immunoprotective and immunodiagnostic potential of T. pallidum
proteins, and clarification of the roles of treponemal
proteins in pathogenesis.