Proteolysis of SpoIVB is a critical determinant in signalling of Pro-sigma(K) processing in Bacillus subtilis

SpoIVB is essential for intercompartmental signalling in the sigma(K)-check point of Bacillus subtilis. SpoIVB is synthesized in the spore chamber and is the signal which activates proteolytic processing of pro-sigma(K) to its mature and active form sigma(K). We show here that SpoIVB is a serine pept idase of the SA clan. Expression of SpoIVB in Escherichia coli has shown th at SpoIVB is able to self-cleave into at least three discrete products, and in vitro studies have shown cleavage in trans. Autoproteolysis of SpoIVB i s tightly linked to the initiation of the two developmental functions of th is protein, signalling of pro-sigma(K) processing and a yet, uncharacterize d, second function which is essential for the formation of heat-resistant s pores. In B. subtilis, SpoIVB is synthesized as a zymogen and is subject to two levels of proteolysis. First, autoproteolysis generating intermediate products, at least one of which is proposed to be the active form, followed by processing by one or more enzymes to smaller species. This could provid e a mechanism for switching off the active SpoIVB intermediate(s) and sugge sts a similarity to other proteolytic cascades such as those found in blood coagulation.