High-temperature requirement A (HtrA) protein has been known as a moonlighting protein that plays dual roles as a molecular chaperone and as a protease. The proteolytic activity of HtrA is switched on at elevated temperatures, whereas the chaperone function predominates at normal temperatures. The temperature-regulated functional switch of HtrA appears to be critical for the control of the stability of cellular proteins, as well as for the elimination of denatured proteins in order to maintain viability. Although certain conformational changes are expected to be concurrent with the functional activation of HtrA proteolysis, the molecular mechanisms inherent to this process have yet to be elucidated. Spin labeling electron paramagnetic resonance and fluorescence spectroscopy experiments on the HtrA from Thermotoga maritima (Tm HtrA) have shown that a helical lid (H(L)) that covers the active site is lifted up to expose the catalytic and substrate-binding sites to the solvent at elevated temperatures, whereas the overall structure is maintained over a wide temperature range. Results indicate that the proteolytic activity of Tm HtrA is turned on by the geometric change occurring around the H(L), resulting in a substrate-accessible path. In conclusion, the functional switch of Tm HtrA is embedded in the sentinel of the H(L) in terms of substrate accessibility.