Identification of APOBEC3G (A3G) as a restriction factor for HIV-1 and subsequent studies have revealed that A3 enzymes play an important role in viral restriction 5, 6. The histidine and cysteine residues coordinate the zinc ion whereas the glutamic acid acts as a proton shuttle during the catalytic deamination reaction. Each catalytic domain contains a highly conserved zinc-dependent deaminase motif comprised of HX 1EX 23- 28CX 2- 4C (where X is any amino acid) 3, 4. A3A, A3C and A3H have a single catalytic domain, whereas A3B, A3D, A3F and A3G have two, N-and C-terminal catalytic domains (NTD and CTD) 2. The APOBEC3 (A3) family of cytidine deaminases in primates is comprised of seven homologous enzymes that are structurally related to the RNA editing enzyme APOBEC1 1. Our findings demonstrate the novel RNA editing function of APOBEC3G and suggest a role for the N-terminal domain in RNA editing. We find that conserved catalytic residues in both cytidine deaminase domains are required for RNA editing. APOBEC3G co-purifies with highly edited mRNA substrates. We verified protein-recoding RNA editing of selected genes including several that are known to be involved in HIV-1 infectivity. The edited cytidines are often flanked by inverted repeats, but are largely distinct from those deaminated by APOBEC3A. We show that APOBEC3G causes site-specific C-to-U editing of mRNAs from over 600 genes. To determine if APOBEC3G is capable of RNA editing, we transiently expressed APOBEC3G in the HEK293T cell line and performed transcriptome-wide RNA sequencing. We recently discovered RNA editing by the single-domain enzyme APOBEC3A in innate immune cells. Although APOBEC3G is known to bind RNAs, APOBEC3G-mediated RNA editing has not been observed. APOBEC3G deaminates single-stranded DNAs via its C-terminal domain, whereas the N-terminal domain is considered non-catalytic. APOBEC3G is a cytidine deaminase with two homologous domains and restricts retroelements and HIV-1.
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