Regulated expression of AmyQ α-amylase of Bacillus amyloliquefaciens was used to examine the capacity of the protein secretion apparatus of B. subtilis. One B. subtilis cell was found to secrete maximally 10 fg of AmyQ per h. The signal peptidase SipT limits the rate of processing of the signal peptide. Another limit is set by PrsA lipoprotein. The wild-type level of PrsA was found to be 2 × 104 molecules per cell. Decreasing the cellular level of PrsA did not decrease the capacity of the protein translocation or signal peptide processing steps but dramatically affected secretion in a posttranslocational step. There was a linear correlation between the number of cellular PrsA molecules and the number of secreted AmyQ molecules over a wide range of prsA and amyQ expression levels. Significantly, even when amyQ was expressed at low levels, overproduction of PrsA enhanced its secretion. The finding is consistent with a reversible interaction between PrsA and AmyQ. The high cellular level of PrsA suggests a chaperone-like function. PrsA was also found to be essential for the viability of B. subtilis. Drastic depletion of PrsA resulted in altered cellular morphology and ultimately in cell death
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