Inhibits HSPA1A chaperone activity by changing the conformation of the ATP-binding domain of HSPA1A and interfering with ATP binding. Interferes with ubiquitination mediated by STUB1 and inhibits chaperone-assisted degradation of immature CFTR. Interacts with the ATP-binding domain of HSPA1A. Detected in a ternary complex containing STUB1, HSPA1A and HSPBP1. Interacts with PGLYRP1; this interaction blocks the cytotoxic activity of the PGLYRP1-HSPA1A complex (PubMed:21247889). Ubiquitous.