The alkaline shock protein Asp23 was identified as an alkaline shock protein that was expressed in a sigmaB-dependent manner in Staphylococcus aureus. Following an alkaline shock Asp23 accumulates in the soluble protein fraction of the S. aureus cell. Asp23 is one of the most abundant proteins in the cytosolic protein fraction of stationary S. aureus cells, with a copy-number of >25000 per cell. A second Asp23-family protein, AmaP, which is encoded within the asp23-operon, is required to localize Asp23 to the cell membrane. The overall function for the family is thus a cell envelope-related one in Gram-positive bacteria.