AHA-1 stimulates the inherent ATPase activity of yeast and human HSP 90 and interacts with the cytoplasmic tail of vesticular stomatitis virus glycoprotein. AHA-1 regulates HSP 90 by influencing the conformational state of the “ATP lid” and consequent N-terminal dimerization. It is crucial for cell viability under non-optimal growth conditions when HSP 90 levels are limiting. AHA-1 is a cytosolic protein and may transiently interact with the endoplasmic reticulum. It can have an affect on one step in the endoplasmic to Golgi trafficking. AHA-1 is expressed in numerous tissues, including brain, heart, skeletal muscle and kidney, and at lower levels in liver and placenta. It is induced by heat shock and treatment with the HSP 90 inhibitor 17-demethoxygeldanamycin.