Reassessing the role of the secreted protease CPAF in Chlamydia trachomatis infection through genetic approaches

Abstract

The secreted Chlamydia protease CPAF cleaves a defined set of mammalian and Chlamydia proteins in vitro. As a result, this protease has been proposed to modulate a range of bacterial and host cellular functions. However, it has recently come into question the extent to which many of its identified substrates constitute bona fide targets of proteolysis in infected host cell rather than artifacts of postlysis degradation. Here, we clarify the role played by CPAF in cellular models of infection by analyzing Chlamydia trachomatis mutants deficient for CPAF activity. Using reverse genetic approaches, we identified two C. trachomatis strains possessing nonsense, loss-of-function mutations in cpa (CT858) and a third strain containing a mutation in type II secretion (T2S) machinery that inhibited CPAF activity by blocking zymogen secretion and subsequent proteolytic maturation into the active hydrolase. HeLa cells infected with T2S(-) or CPAF(-) C. trachomatis mutants lacked detectable in vitro CPAF proteolytic activity and were not defective for cellular traits that have been previously attributed to CPAF activity, including resistance to staurosporine-induced apoptosis, Golgi fragmentation, altered NFκB-dependent gene expression, and resistance to reinfection. However, CPAF-deficient mutants did display impaired generation of infectious elementary bodies (EBs), indicating an important role for this protease in the full replicative potential of C. trachomatis. In addition, we provide compelling evidence in live cells that CPAF-mediated protein processing of at least two host protein targets, vimentin filaments and the nuclear envelope protein lamin-associated protein-1 (LAP1), occurs rapidly after the loss of the inclusion membrane integrity, but before loss of plasma membrane permeability and cell lysis. CPAF-dependent processing of host proteins correlates with a loss of inclusion membrane integrity, and so we propose that CPAF plays a role late in infection, possibly during the stages leading to the dismantling of the infected cell prior to the release of EBs during cell lysis.

Keywords: Chlamydia mutants; live cell imaging; pathogenesis; proteolysis.