Enzymatic activity of cGAS in the presence of three types of DNAs: limited cGAS stimulation by single-stranded HIV-1 SL2 DNA.

Cyclic GMP-AMP (cGAMP) synthase (cGAS) is activated by binding to DNA. Activated cGAS produces 2'3'-cGAMP, which subsequently binds to the adaptor protein STING (stimulator of interferon genes). This interaction triggers the cGAS/STING signaling pathway, leading to the production of type I interferons. Three types of DNA, namely double-stranded DNA longer than 40 base pairs, a 70-nucleotide single-stranded HIV-1 DNA known as SL2, and Y-form DNA with unpaired guanosine trimers (G3 Y-form DNA), induce interferon production by activating cGAS/STING signaling. However, the extent of cGAS activation by each specific DNA type remains unclear. The comparison of cGAS stimulation by various DNAs is crucial for understanding the mechanisms underlying cGAS-mediated type I interferon production in the innate immune response. Here, we revealed that cGAS produces 2'3'-cGAMP at a significantly lower rate in the presence of single-stranded SL2 DNA than in the presence of double-stranded DNA or G3 Y-form DNA. Furthermore, the guanine-to-cytosine mutations and the deletion of unpaired guanosine trimers significantly reduced the 2'3'-cGAMP production rate and the binding of cGAS to Y-form DNA. These studies will provide new insights into the cGAS-mediated DNA-sensing in immune response.