The Yersinia plasmid-encoded Yop virulon enables extracellular adhering bacteria to deliver toxic effector
proteins inside their target cells. It includes a
type III secretion system (Ysc), at least two translocator
proteins (YopB, YopD), and a set of intracellular Yop effectors (YopE, YopH, YopO, YopM, and YopP).
Infection of macrophages with a wild-type strain leads to low levels of
tumor necrosis factor alpha (
TNF-alpha) release compared to
infection with plasmid-cured strains, suggesting that the virulence plasmid encodes
a factor impairing the normal
TNF-alpha response of infected macrophages. This effect is correlated with the inhibition of the macrophage
mitogen-activated protein kinase (MAPK) activities. To identify the Yop
protein responsible for the suppression of
TNF-alpha release, we infected J774A.1 and PU5-1.8 macrophages with a battery of knockout Yersinia enterocolitica mutants and we quantified the
TNF-alpha released. Mutants affected in secretion (yscN), in translocation (yopB and yopD), or in synthesis of all the known Yop effectors (yopH, yopO, yopP, yopE, and yopM polymutants) were unable to block the
TNF-alpha response of the macrophages. In contrast, single yopE, yopH, yopO, and yopM mutants behaved like the wild-type strain. A yopP mutant elicited elevated
TNF-alpha release, and complementation of the yopP mutant or the yop effector polymutant strain with yopP alone led to a drop in
TNF-alpha release. In addition, YopP was also responsible for the inhibition of the extracellular signal-regulated kinase2 (ERK2) and
p38 MAPK activities. These results show that YopP is the Yop effector responsible for the Yersinia-induced suppression of
TNF-alpha release by infected macrophages.