Numerous investigations have shown that 70-kDa
heat shock protein (Hsp70) homologs interact tightly with hydrophobic
proteins and functionally assist
proteins in membranous organelles and environments. One such
protein is the Chlamydia trachomatis Hsp70 that is associated with isolated outer membrane complexes of infectious elementary bodies (EB). Previous observations have indicated that chlamydial Hsp70 plays a role in EB attachment to, or entry into, endometrial epithelial cells. In this study, immunofluorescence microscopy and transmission electron microscopy observations showed that chlamydial Hsp70 is not a surface-displayed
ligand on purified EB. However, brief exposure of EB to the
thiol reducing agent dithiothreitol (DTT) led to surface accessibility of the Hsp70 substrate-binding domain. Reduction of the highly
disulfide-cross-linked EB outer
membrane proteins with DTT resulted in a decrease in EB attachment and infectivity. Interestingly, exposure of EB to the membrane-impermeable
thiol-alkylating
reagent 5,5'-dithiobis(2-nitrobenzoic acid) enhanced attachment but compromised infectivity, suggesting that EB outer
membrane proteins must be reduced for entry and productive
infection. Together, our data suggest that (i) the structural integrity of the EB outer membrane, maintained by
protein disulfide bonds, is important during the initial stages of attachment; (ii) reduction occurs within the localized microenvironment of host cell surfaces once intimate contact is established between EB and host cells; and (iii) subsequent conformational changes in EB ultrastructure allow productive
infection in host cells. The accessibility of the Hsp70 substrate-binding domain may support the hypothesis that this
protein plays a role in events following the initial stage of attachment instead of serving as a primary, surface-displayed adhesin.