Membrane‐bound proteinases of Halobacterium halobium

Cell envelopes of Halobacterium halobium contain proteolytic activities, which cleave 125 I‐insulin and 14 C‐casein with pH‐optima in the alkaline range. About 50% of the intracellular proteases are strongly attached to the cell envelopes and cannot be removed by washing with 25% sodium chloride solution. This high amount of membrane‐bound proteases is independent of the kind of cultivation. The main part of the envelope‐bound proteolytic activity is attributed to metalloproteases, with EDTA and o‐phenanthroline causing inhibition. Serine and cysteine protease inhibitors had no influence. In density gradients (consisting of Ficoll and 20% sodium chloride) proteolytic activities could be detected in two distinct bands, a minor band at a density of 1.165 and the main proteolytic activity at 1.145 g/cm 3 . When the cell wall was removed from the other cell envelope constituents by EDTA treatment, a single band with proteolytic activity in the density gradient at 1.145 g/cm 3 was detectable. Lubrol, Brij 35, and octyl glucoside were used for the solubilization of the membrane‐bound proteases. The greatest part of them could be solubilized using 0.2% octyl glucoside. 14 C‐labelled bacteriorhodopsin was not cleaved by any of the intracellular or extracellular proteinases.