This implies that in the absence of RACK1, eukaryotic cells are protected from the Yersinia antiphagocytic attack. Notably, the absence of RACK1 could not protect RACK RNAi cells from 3,4,5-Trimethoxyphenylacetic acid antiphagocytosis induced by the yopK null mutant or the yopKAA mutant. In contrast, RACK1 RNAi cells were resistant towards the antiphagocytic activity of a trans-complemented yopK null mutant. Pathogenic Yersinia species utilize a powerful T3SS to deliver effector proteins into the interior of the targeted host cells. A set of translocator proteins form a pore in the host cell membrane through which these effectors are thought to gain access into the host cell cytosol. This virulence mechanism allows these bacteria to purposefully block innate immune cell defense mechanisms such as phagocytosis so they can proliferate in lymphatic tissue. However, the phagocytic process is initiated as soon as a bacterium comes in contact with a target cell and completed within minutes after cell contact. Therefore, it is assumed that pathogenic Yersinia battle to impair this process via a very rapid and precise mechanism for effector delivery and action. In line with this, we have previously shown that Yop effector deployment and intracellular activity can be measured within 30 seconds after the bacterium makes contact with a host cell. Nevertheless, extreme rapidity alone would not be sufficient; individual Yops would also need targeting to the precise site of action. Regulation of this process in the bacterium occurs on several levels; this includes coordinating target cell contact with induction of gene expression, subsequent feed-back repression, and via control of effector translocation. We have presented data in this study to support a mechanism that guarantees productive antiphagocytic effector translocation leading to an instant blockage of phagocytosis. It relies upon an interaction in the host cell between YopK, which is associated with the pore complex, and the eukaryotic protein RACK1. We observed that RACK1 was specifically required for antiphagocytosis. This was based on the finding that host cells downregulated for RACK1 expression engulfed the Folinic acid calcium salt pentahydrate pathogen to a much greater extent compared to cells with normal RACK1 expression, implying that these RACK1 RNAi cells are resistant to the antiphagocytic activity of the T3SS. This was quite unexpected; making it the first reported cell line to be impervious to the action of the Yersinia antiphagocytic machinery. Importantly, virulence effectors were translocated to the same extent into cells regardless of their RACK1 expression level and these intracellular effectors still induced a normal cytotoxic response in both cell lines. This means two things; firstly, translocation of effectors and the process of cytotoxicity induction do not require RACK1, and secondly, antiphagocytosis and cytotoxicity are clearly two distinct events during cell infection. Another striking feature was that the ability of the pathogen to mediate antiphagocytosis towards RACK1 RNAi cells diminished significantly when YopK was present. Notably, this failure to inactivate the critical cellular targets and interrupt phagocytosis occurred despite efficient antiphagocytic Yop effector translocation into the target cells. Hence, the discrepancy between Yersinia YopK+ bacteria inducing an uncompromising cytotoxic response on the one hand, but failing to establish antiphagocytosis on the other, suggests that the latter process requires specific features of the effector translocation process that are overseen by YopK. This agrees with our previous results concerning the antiphagocytic effector and major cytotoxin YopE; certain yopE point mutants exhibited attenuated virulence but still gave rise to cytotoxicity in HeLa cells. Thus, the cytotoxic effect is actually a secondary event that occurs only after prolonged infection and is dispensable for in vivo virulence. On the other hand, there appears to be a direct correlation between the ability of Y. pseudotuberculosis to resist phagocytosis and to cause systemic infections in mice.