Structural basis for anti-CRISPR repression mediated by bacterial operon proteins Aca1 and Aca2

It has been shown that phages have evolved anti-CRISPR (Acr) proteins to inhibit host CRISPR-Cas systems. Most acr genes are located upstream of anti-CRISPR-associated (aca) genes, which is instrumental for identifying these acr genes. Thus far, eight Aca families (Aca1-Aca8) have been identified, all proteins of which share low sequence homology and bind to different target DNA sequences. Recently, Aca1 and Aca2 proteins were discovered to function as repressors by binding to acr-aca promoters, thus implying a potential anti-anti-CRISPR mechanism. However, the structural basis for the repression roles of Aca proteins is still unknown. Here, we elucidated apo-structures of Aca1 and Aca2 proteins and their complex structures with their cognate operator DNA in two model systems, the Pseudomonas phage JBD30 and the Pectobacterium carotovorum template phage ZF40. In combination with biochemical and cellular assays, our study unveils dimerization and DNA-recognition mechanisms of Aca1 and Aca2 family proteins, thus revealing the molecular basis for Aca1-and Aca2-mediated anti-CRISPR repression. Our results also shed light on understanding the repression roles of other Aca family proteins and autoregulation roles of acr-aca operons.

Molecular Characterization of Adenylyl Cyclase Complex Proteins Using Versatile Protein-Tagging Plasmid Systems in Cryptococcus neoformans

In this study, we aimed to generate a series of versatile tagging plasmids that can be used in diverse molecular biological studies of the fungal pathogen Cryptococcus neoformans. We constructed 12 plasmids that can be used to tag a protein of interest with a GFP, mCherry, 4×FLAG, or 6×HA, along with nourseothricin-, neomycin-, or hygromycin-resistant selection markers. Using this tagging plasmid set, we explored the adenylyl cyclase complex (ACC), consisting of adenylyl cyclase (Cac1) and its associated protein Aca1, in the cAMP-signaling pathway, which is critical for the pathogenicity of C. neoformans. We found that Cac1-mCherry and Aca1-GFP were mainly colocalized as punctate forms in the cell membrane and nonnuclear cellular organelles. We also demonstrated that Cac1 and Aca1 interacted in vivo by coimmunoprecipitation, using Cac1-6×HA and Aca1-4×FLAG tagging strains. Bimolecular fluorescence complementation further confirmed the in vivo interaction of Cac1 and Aca1 in live cells. Finally, protein pull-down experiments using aca1Δ::ACA1-GFP and aca1Δ::ACA1- GFP cac1Δ strains and comparative mass spectrometry analysis identified Cac1 and a number of other novel ACC-interacting proteins. Thus, this versatile tagging plasmid system will facilitate diverse mechanistic studies in C. neoformans and further our understanding of its biology.