Inter-molecular degradation of signal peptidase I in vitro.

Highly purified preparations of signal peptidase I (36 kDa) were found to undergo an apparent inter-autocatalytic degradation at 4 degrees C and 37 degrees C. The disappearance of the 36 kDa protein coincided with the stable appearance of a 31 kDa and a 5 kDa species. Amino-terminal sequencing of the 31 kDa product indicated a site specific cleavage following Ala38-Gln-Ala of signal peptidase I. The 31 kDa fragment was purified and shown to have 100-fold less activity than the native enzyme, with pre-maltose binding protein as a substrate.