Secretion of Tat‐dependent halolysin SptA capable of autocatalytic activation and its relation to haloarchaeal growth

Halolysins are Tat‐dependent extracellular subtilases of haloarchaea. Whether halolysins can be activated before transport across the cytoplasmic membrane in a folded state and how haloarchaea minimize the risk of intracellular activation of halolysins and proteolysis of cellular proteins are unknown. Here, we report that both the precursor and proform of halolysin SptA from Natrinema sp. J7‐2 mature autocatalytically, and the SptA maturation proceeds less efficiently in the presence of KCl than NaCl. When produced in Haloferax volcanii, most SptA molecules are secreted into the culture medium, but a small number of molecules can be activated intracellularly, affecting the cell's growth. Furthermore, retardation of SptA secretion in Hfx. volcanii via mutation of the Tat signal peptide leads to intracellular accumulation of the active enzyme and subsequent cell death. Although the Sec signal peptide can mediate SptA secretion in Hfx. volcanii, the secreted protein undergoes proteolysis. In Natrinema sp. J7‐2, SptA is secreted primarily during stationary phase, and the intracellular accumulation of mature enzyme occurs during the stationary and death phases. The growth phase‐dependent synthesis of SptA, highly efficient secretion system, and high intracellular KCl concentration, contribute to the suppression of premature activation of this enzyme in Natrinema sp. J7‐2.