Chaperone of the type III secretion (T3S) injectisome that translocates effector toxins into host cells, facilitating the establishment and dissemination of infection. Prevents premature polymerization of PscF, along with PscE, within the cytoplasm (PubMed:20494986). Required for type III secretion needle assembly. Also required for cytotoxicity by influencing PscF levels. Forms a stable heterotrimeric complex with PscE and PscF in the cytoplasm (PubMed:17470796). Co-stabilized by PscE. Deletion mutant looses the capacity to lyse macrophages, indicating the absence of a functional T3SS. Belongs to the YscG family.