Protein Transport Pathways in Bacillus subtilis: a Genome-Based Road Map

This chapter on protein transport pathways in Bacillus subtilis: a genome-based road map, discusses the properties of distinct classes of amino-terminal signal peptides, which label proteins for passage through the various pathways, as well as the major components of each pathway. The cell wall of B. subtilis contains about 9% of the total cellular protein content, analogous to the periplasm of gram-negative eubacteria. The proteins retained in the wall include DNases, RNases, proteases, penicillin-binding proteins, and cell wall hydrolases. Presumably, to prevent their loss in the environment, the mature parts of 12 B. subtilis proteins with cleavable signal peptides, such as LytB-F, SleB, and WapA, contain potential cell wall retention signals, which consist of a variable number of repeated domains with affinity for certain wall components, such as teichoic acids. Eubacterial Sec-dependent protein export machineries are composed of six types of components: (i) cytoplasmic chaperones or targeting factors, (ii) a translocation motor (SecA), (iii) components of the translocation channel (SecYEG, SecDF-YajC, YidC), (iv) SPases, (v) SPPases, and (vi) folding catalysts. The best-characterized secretion-specific targeting factor of B. subtilis is the GTPase Ffh (fifty-four homologue), a homologue of the 54-kDa subunit of the eukaryotic signal recognition particle (SRP). The expression of components of various protein transport pathways in B. subtilis is regulated in a complex manner, depending on the availability of nutrients, growth phase, and cell density.