RNA interference screen identifies AbI kinase and PDGFR signaling in Chlamydia trachomatis entry

To elucidate the mechanisms involved in early events in Chlamydia trachomatis infection, we conducted a large scale unbiased RNA interference screen in Drosophila melanogaster S2 cells. This allowed identification of candidate host factors in a simple non-redundant, genetically tractable system. From a library of 7,216 double stranded RNAs (dsRNA), we identified,226 host genes, including two tyrosine kinases, Abelson (AbI) kinase and PDGF- and VEGF-receptor related (Pvr), a homolog of the Platelet-derived growth factor receptor ( PDGFR). We further examined the role of these two kinases in C. trachomatis binding and internalization into mammalian cells. Both kinases are phosphorylated upon infection and recruited to the site of bacterial attachment, but their roles in the infectious process are distinct. We provide evidence that PDGFR beta may function as a receptor, as inhibition of PDGFR beta by RNA interference or by PDGFR beta neutralizing antibodies significantly reduces bacterial binding, whereas depletion of AbI kinase has no effect on binding. Bacterial internalization can occur through activation of PDGFR beta or through independent activation of AbI kinase, culminating in phosphorylation of the Rac guanine nucleotide exchange factor (GEF), Vav2, and two actin nucleators, WAVE2 and Cortactin. Finally, we show that TARP, a bacterial type III secreted actin nucleator implicated in entry, is a target of AbI kinase. Together, our results demonstrate that PDGFR beta and AbI kinases function redundantly to promote efficient uptake of this obligate intracellular parasite.