Global Ion Suppression Limits the Potential of Mass Spectrometry Based Phosphoproteomics.

Mass spectrometry based proteomics has become the method of choice for pinpointing and monitoring thousands of post-translational modifications, predominately phosphorylation sites, in cellular signaling studies. Critical for achieving this analytical depth is the enrichment of phosphorylated peptides prior to liquid chromatography-mass spectrometry (MS) analysis. Despite the high prevalence of this modification, the numbers of identified phosphopeptides lag behind those achieved for unmodified peptides, and the cause for this still remains controversial. Here, we use an effective phosphatase protocol that considerably improves global ionization efficiency and, therefore, the overall sensitivity and coverage of standard phosphoproteomics studies. We demonstrate the power of our method on the model system of Salmonella-infected macrophages by extending the current quantitative picture of immune signaling pathways involved in infection. In combination with sensitive, label-free targeted MS for phosphorylation site validation, our approach is ideally suited to exploring cellular phosphorylation based signaling networks in high detail.

Detecting Release of Bacterial dsDNA into the Host Cytosol Using Fluorescence Microscopy.

Recognition of pathogens by the innate immune system relies on germline-encoded pattern recognition receptors (PRRs) that recognize unique microbial molecules, so-called pathogen-associated molecular patterns (PAMPs). Nucleic acids and their derivatives are one of the most important groups of PAMPs, and are recognized by a number of surface-associated as well as cytosolic PRRs. Cyclic GMP-AMP synthase (cGAS) recognizes the presence of pathogen- or host-derived dsDNA in the cytosol and initiates type-I-IFN production. Here, we describe a methodology that allows for evaluating the association of cGAS with released bacterial dsDNA during Francisella novicida infection of macrophages, by fluorescence confocal microscopy. This method can be adapted to the study of cGAS-dependent responses elicited by other intracellular bacterial pathogens and in other cell types.