An ATP-independent periplasmic chaperone, decreases protein aggregation and helps protein refolding. Binds substrate over a large region of its convex inner surface (PubMed:21317898, PubMed:24497545). Substrate protein folds while it is bound to chaperone (PubMed:26619265). Increasing Spy flexibility increases its substrate affinity and overall chaperone activity (shown for 3 different substrates) (PubMed:24497545). Protects proteins in vitro against tannin inactivation; tannins have antimicrobial activity (PubMed:21317898). Overexpression enhances the stability of otherwise unstable periplasmic proteins (PubMed:21317898). Homodimer. Produced abundantly in spheroplasts, not detected in intact cells. Induced by many sorts of periplasmic stress. Produced upon spheroplast formation, not induced by growth in 0.5 M sucrose or 0.35 M NaCl, nor by mutants that have a leaky periplasmic space (at protein level) (PubMed:9068658). Induction is partially dependent on the Cpx envelope stress response encoded by cpxA-cpxR; does not depend on sigma factor E (rpoE) (PubMed:10972835). Induction is also partially dependent on the Bae envelope stress response encoded by baeS-baeR; cpx and baeS effects are independent (PubMed:12354228). Overexpression of P pili protein PapG induces spy expression via both Cpx and Bae, overexpression of outer membrane protein NlpE induces spy via only Cpx whereas indole induces spy expression only via Bae (PubMed:12354228). Induced by 1 mM zinc (at protein level) (PubMed:9694902), via Bae, which also regulates ethanol induction of spy (PubMed:24999585). Induced by copper via Cpx (PubMed:24999585). Induced by tannic acid (to 25% of total periplasmic protein), butanol (to 20% total periplasmic protein) and ethanol (to 5% total periplasmic protein) (at protein level) (PubMed:21317898). Has an elongated cradle shape. No observed growth phenotype (PubMed:9068658). Increased levels of degP and rpoH transcription; may induce the sigma factor E regulon (rpoE) (PubMed:10972835). Loss of about 50% of periplasmic alkaline phosphatase activity; very sensitive to tannic acid (PubMed:21317898). Belongs to the CpxP/Spy family.