Distinct Domains Contribute to the Subcellular Localization of Human cGAS in Yeast

Cyclic GMP-AMP synthase (cGAS) functions as a DNA sensor in the cytoplasm, triggering immune responses, but it is also translocated to the nucleus, where it is kept catalytically inactive. It consists of an unstructured N-terminal domain of around 160 amino acids, and a larger C-terminal fold comprising the catalytic and DNA-binding domains. Subcellular localization of cGAS is thought to play a key role in its regulation. Here, we make use of heterologous expression in the eukaryotic model Saccharomyces cerevisiae to study cGAS localization in a neutral cellular environment. cGAS-eGFP was mostly found in aggregates at the endoplasmic reticulum-mitochondria encounter structure (ERMES) and juxtanuclear protein quality compartments (JUNQs), although some cells displayed an association between cGAS-eGFP and the plasma membrane. The N-terminus of cGAS fused to eGFP was unable to associate with the plasma membrane by itself, but its deletion dramatically promoted nuclear localization of cGAS-eGFP and decreased cytoplasmic aggregates. A mutant in the DNA-binding Zn-thumb motif of cGAS also showed a more prominent nuclear localization. Thus, both the N-terminal and C-terminal domains of cGAS seem to cooperate to prevent nuclear localization and to maintain cytoplasmic reservoirs of the protein. Heterologous cGAS expression in yeast is a valuable tool for modeling aspects of its subcellular localization and aggregative features.