Aqualysin I is a thermophilic alkaline serine protease. Exhibits low specificity toward esters of amino acids with small hydrophobic or aromatic residues at the P1 position. Optimum temperature is 80 degrees Celsius for caseinolytic activity. Secreted from the early stationary phase until the time the cells cease to grow. The N- and C-terminal Pro-sequences are successively removed through the proteolytic activity of PstI itself. The C-terminal Pro-sequence is required for translocation of the proteases across the outer membrane. Belongs to the peptidase S8 family.