Release of the cell-envelope protease PrtS in the growth medium of Streptococcus thermophilus 4F44

PrtS is the sole cell envelope protease (CEP) characterized in Streptococcus thermophilus. It is believed that it is anchored to the cell wall by sortase A (SrtA) through the LPXTG motif present at its C-terminus. Two soluble proteases corresponding to PrtS in its proenzyme and mature form were detected in the supernatant of S. thermophilus strain 4F44. In this strain, 60% of the PrtS molecules are anchored to the cell wall and 40% released in the medium. Such a release might result from a partial deficiency in the strain 4F44 of SrtA, even if its sequence slightly differs from that of S. thermophilus strain LMD-9, in which PrtS is anchored. Indeed, the presence of an intact LPXTG motif at the C-terminus of the released proteases showed that the linking process driven by SrtA did not occur and these proteases were not released by proteolysis after their anchoring.