An Anaplasma phagocytophilum T4SS effector, AteA, is essential for tick infection

Anaplasma phagocytophilum AFAP targets the host nucleolus and inhibits induced apoptosis

Anaplasma phagocytophilum, the etiologic agent of human granulocytic anaplasmosis (HGA), is an obligate intracellular Gram-negative bacterium. During infection, A. phagocytophilum transfers its type IV secretion system (T4SS) effector proteins into host cells to manipulate cellular processes. AFAP (an actin filament-associated Anaplasma phagocytophilum protein) was identified as a T4SS effector protein and found to interact with the host nucleolin, as described in a previous study. In this study, proteomic analysis was performed to extensively identify AFAP-interacting proteins in host cells and analyze the potential role of AFAP in modulating host cellular processes. A total of 586 host proteins were identified interacting with AFAP by data-independent acquisition mass spectrometry and annotated to 501 Gene Ontology (GO) terms, with the significantly over-represented ones related to ribosomes, nucleolus, DNA binding, and rRNA metabolic process. Given the role of the nucleolus in cellular stress response, the targeting of AFAP to the nucleolus, and the identification of dozens of AFAP-interacting proteins that were annotated to the GO term (GO:0072331, signal transduction by p53 class mediator), the role of AFAP in modulating host apoptosis was determined. AFAP was found capable of inhibiting induced apoptosis. Thus, the proteomic analysis of AFAP-interacting proteins and determination of AFAP with anti-apoptotic activity may help elucidate the role of this T4SS effector protein in HGA pathogenesis.

Anaplasma phagocytophilum effector EgeA facilitates infection by hijacking TANGO1 and SCFD1 from ER-Golgi exit sites to pathogen-occupied inclusions

The obligatory intracellular bacterium Anaplasma phagocytophilum causes human granulocytic anaplasmosis, an emerging zoonosis. Anaplasma has limited biosynthetic and metabolic capacities, yet it effectively replicates inside of inclusions/vacuoles of eukaryotic host cells. Here, we describe a unique Type IV secretion system (T4SS) effector, ER-Golgi exit site protein of Anaplasma (EgeA). In cells infected by Anaplasma, secreted native EgeA, EgeA-GFP, and the C-terminal half of EgeA (EgeA-C)-GFP localized to Anaplasma-containing inclusions. In uninfected cells, EgeA-C-GFP localized to cis-Golgi, whereas the N-terminal half of EgeA-GFP localized to the ER. Pull-down assays identified EgeA-GFP binding to a transmembrane protein in the ER, Transport and Golgi organization protein 1 (TANGO1). By yeast two-hybrid analysis, EgeA-C directly bound Sec1 family domain-containing protein 1 (SCFD1), a host protein of the cis-Golgi network that binds TANGO1 at ER-Golgi exit sites (ERES). Both TANGO1 and SCFD1 localized to the Anaplasma inclusion surface. Furthermore, knockdown of Anaplasma EgeA or either host TANGO1 or SCFD1 significantly reduced Anaplasma infection. TANGO1 and SCFD1 prevent ER congestion and stress by facilitating transport of bulky or unfolded proteins at ERES. A bulky cargo collagen and the ER-resident chaperon BiP were transported into Anaplasma inclusions, and several ER stress marker genes were not up-regulated in Anaplasma-infected cells. Furthermore, EgeA transfection reduced collagen overexpression-induced BiP upregulation. These results suggest that by binding to the two ERES proteins, EgeA redirects the cargo-adapted ERES to pathogen-occupied inclusions and reduces ERES congestion, which facilitates Anaplasma nutrient acquisition and reduces ER stress for Anaplasma survival and proliferation.