PDZ domains are one of the largest families of short linear motif (or peptide) binding domains in the human proteome. These scaffolding domains are important in signaling and trafficking pathways, such as in the formation of tight junctions or in the postsynaptic density of neurons. PDZ domains are also targeted by several pathogenic viral proteins, including human papillomavirus (HPV), influenza, hepatitis, rabies, coronaviruses, among others. Previously, we investigated the specificity determinants of modulator, or non‐motif, residues for two PDZ binding motifs (or PBMs) from the cystic fibrosis transmembrane conductance regulator (CFTR) and HPV16 E6 proteins to better understand differences in relative promiscuity between these similar sequences. To test whether viral PDZ‐binding motifs are inherently more promiscuous than endogenous targets, we measured binding affinities for 7 endogenous and 7 viral PBMs across 8 well‐studied Class I PDZ domains. Fluorescence anisotropy assays revealed no significant difference in relative binding affinities between viral and endogenous PBMs, indicating that viral versus endogenous origin alone does not explain PBM promiscuity. Our results were consistent with available data from high throughput holdup assays. Taken together, our data support previously reported sequence hotspots, whereby certain PBMs are recognized by large numbers of PDZ domains, likely due to a combination of widely favorable modulator residues.
Cole D. Masuga, Caroline Ceravolo, Elise F. Tahti et al.· Protein Science· 0 citations
Bacterial sortase enzymes are cysteine transpeptidases at the surface of Gram-positive bacteria that ligate substrates to the cell wall. In addition, these enzymes are powerful tools in protein engineering applications via sortase-mediated ligation (SML) due to their covalent attachment of two substrates, with one containing a pentapeptide recognition motif with sequence LPXTG, where X=any amino acid, and the second, an N-terminal glycine. The class A sortase from Staphylococcus aureus (saSrtA) was the first to be identified, and over 25 years later, the most widely used SML variants continue to be derivatives of a directed-evolution-identified pentamutant of saSrtA, or saSrtA5M. We previously characterized P94, a position mutated in saSrtA5M that interacts directly with a structurally conserved loop (the β7-β8 loop) near the active site of wild-type saSrtA only in the inactive conformation. This work revealed that the single P94X mutation dramatically affects relative saSrtA activity, as well as specificity for the P2 (or X) position in the LPXTG recognition motif. This is largely driven by Km effects. Here, we further interrogated P94 by probing structural changes in the active, apo state of saSrtA in the presence of the P94D mutation, as well as via mutations in Y187, the β7-β8 loop residue hypothesized to interact directly with P94. The saSrtA enzyme is allosterically activated by calcium; therefore, we were interested if P94D would induce structural changes in the calcium-bound apo enzyme. We used 1H-15N NMR experiments to compare spectra between enzymatically inactive variants of saSrtA with and without the P94D mutation. We also used NMR to calculate relative binding affinities for a pentapeptide substrate to these variants, as well as enzymatically inactive saSrtA5M. Our NMR data, in combination with enzymatic assays using active variants confirmed differences in the active, apo states of these enzymes. Overall, this work provides additional atomic detail regarding the importance of the P94 residue in saSrtA substrate recognition.
Erich G. Walkenhauer, Noah Cox-Tigre, Manish Chaubey et al.· bioRxiv· 0 citations