Required for surface presentation of invasion plasmid antigens. Chaperone specialized in the storage of effectors within the bacterial cytoplasm, maintaining them in a secretion-competent state, and allowing their immediate delivery to target cells upon contact of the bacterium with the host cells. Has been shown to chaperone IpaA, IpgB1, OspC3 and probably also OspB. Homodimer. Belongs to the SpaK family.