Distinct Contribution of the HtrA Protease and PDZ Domains to Its Function in Stress Resilience and Virulence of Bacillus anthracis

Abstract

Anthrax is a lethal disease caused by the Gram-positive spore-producing bacterium <i>Bacillus anthracis</i>. We previously demonstrated that disruption of <i>htrA</i> gene, encoding the chaperone/protease HtrA_BA (High Temperature Requirement A of <i>B. anthracis</i>) results in significant virulence attenuation, despite unaffected ability of Δ<i>htrA</i> strains (in which the <i>htrA</i> gene was deleted) to synthesize the key anthrax virulence factors: the exotoxins and capsule. <i>B. anthracis</i> Δ<i>htrA</i> strains exhibited increased sensitivity to stress regimens as well as silencing of the secreted starvation-associated Neutral Protease A (NprA) and down-modulation of the bacterial S-layer. The virulence attenuation associated with disruption of the <i>htrA</i> gene was suggested to reflect the susceptibility of Δ<i>htrA</i> mutated strains to stress insults encountered in the host indicating that HtrA_BA represents an important <i>B. anthracis</i> pathogenesis determinant. As all HtrA serine proteases, HtrA_BA exhibits a protease catalytic domain and a PDZ domain. In the present study we interrogated the relative impact of the proteolytic activity (mediated by the protease domain) and the PDZ domain (presumably necessary for the chaperone activity and/or interaction with substrates) on manifestation of phenotypic characteristics mediated by HtrA_BA. By inspecting the phenotype exhibited by Δ<i>htrA</i> strains <i>trans</i>-complemented with either a wild-type, truncated (ΔPDZ), or non-proteolytic form (mutated in the catalytic serine residue) of HtrA_BA, as well as strains exhibiting modified chromosomal alleles, it is shown that (i) the proteolytic activity of HtrA_BA is essential for its N-terminal autolysis and subsequent release into the extracellular <i>milieu</i>, while the PDZ domain was dispensable for this process, (ii) the PDZ domain appeared to be dispensable for most of the functions related to stress resilience as well as involvement of HtrA_BA in assembly of the bacterial S-layer, (iii) conversely, the proteolytic activity but not the PDZ domain, appeared to be dispensable for the role of HtrA_BA in mediating up-regulation of the extracellular protease NprA under starvation stress, and finally (iv) in a murine model of anthrax, the HtrA_BA PDZ domain, was dispensable for manifestation of <i>B. anthracis</i> virulence. The unexpected dispensability of the PDZ domain may represent a unique characteristic of HtrA_BA amongst bacterial serine proteases of the HtrA family.