APOBEC1 COMPLEMENTATION FACTOR; A1CF

Alternative titles; symbols

• ACF
• APOBEC1-STIMULATING PROTEIN; ASP

HGNC Approved Gene Symbol: A1CF

Cytogenetic location: 10q11.23 Genomic coordinates (GRCh38): 10:50,799,408-50,885,665 (from NCBI)

▼ Description

A1CF is an RNA-binding protein required by the catalytic protein APOBEC1 (600130) for C-to-U editing of the apolipoprotein B (APOB; 107730) transcript (Mehta et al., 2000).

▼ Cloning and Expression

By database analysis and screening of a human cDNA array, Mehta et al. (2000) cloned A1CF, which they called ACF. The deduced 586-amino acid protein has a calculated molecular mass of 64.2 kD. The N-terminal region of ACF contains 3 nonidentical RNA recognition motifs (RRMs). The C-terminal region of ACF contains a cluster of 6 RG dipeptides. Northern blot analysis detected ACF transcripts of 9.5 and 2.2 kb in human liver, kidney, and pancreas. PCR analysis showed that ACF was variably expressed in most human tissues.

Lellek et al. (2000) purified A1cf, which they called Asp, as an Apobec1-stimulating protein from rat liver nuclei. Using PCR, they cloned 2 ASP splice variants from a human liver cDNA library and a human small intestine cDNA pool. The intestine-derived cDNA lacked 24 nucleotides compared with the liver-derived cDNA. The predicted proteins contain 586 and 594 amino acids and have calculated molecular masses of 64.3 and 65.2 kD, respectively. ASP contains 3 N-terminal RNA-binding domains (RBDs) and a putative C-terminal double-stranded RBD, as well as a putative N-terminal nuclear localization signal and 2 putative tyrosine phosphorylation sites. Northern blot analysis detected ASP transcripts of about 2.0 and 8.0 kb in human liver and kidney.

By genomic sequence analysis and PCR of human liver, small intestine, and kidney, Henderson et al. (2001) identified multiple ACF splice variants, including the variants reported by Lellek et al. (2000) that include or lack 24 nucleotides in exon 12. The variants showed tissue-specific expression.

▼ Gene Structure

Henderson et al. (2001) determined that the A1CF gene spans approximately 80 kb and contains 15 exons. Exons 1 to 3 are noncoding. The region upstream of exon 1 contains a putative TATA box and multiple binding sites for transcription factors.

▼ Mapping

By database analysis, Henderson et al. (2001) mapped the A1CF gene to chromosome 10q, between markers D10S604 and D10S220.

Dance et al. (2002) reported that the A1CF gene maps to chromosome 10q11.21.

▼ Gene Function

Using ultraviolet cross-linking and immunoprecipitation experiments with recombinant proteins, Mehta et al. (2000) showed that human ACF interacted with APOBEC1, bound APOB mRNA, and complemented APOBEC1 for specific C-to-U editing of APOB. Editing of APOB mRNA by APOBEC1 and ACF was dependent on the 11-nucleotide mooring sequence downstream of the editing site. Editing of exogenous APOB was abolished in rat liver extracts by immunodepletion of Acf alone, but it could be restored by addition of Acf and Apobec1. In vivo analysis corroborated the in vitro results, as Acf and Apob mRNA interacted in nuclear extracts from a rat hepatoma cell line.

By analysis with recombinant proteins, Lellek et al. (2000) showed that ASP and APOBEC1 efficiently reconstituted APOB mRNA editing in vitro. In rat liver, Asp also associated with Ksrp (603445), but recombinant KSRP alone had no ability to stimulate APOBEC1 to edit APOB mRNA. The authors suggested that KSRP may confer stability to the editing enzyme complex.

By in vitro analysis, Henderson et al. (2001) found that approximately 10 to 25% of human ACF spice variants encode nonfunctional proteins. Examination of mRNAs from a human colon cancer-derived cell line and fetal and adult human small intestine showed that an observed developmental increase in APOB mRNA editing was unlikely to be accounted for by a transcriptional increase in ACF.

Dance et al. (2002) showed that the products of the ACF variants containing or lacking the 24 nucleotides in exon 12 were simultaneously expressed in liver and intestinal cells and supported equivalent levels of APOB mRNA editing.

▼ Animal Model

Blanc et al. (2005) found that mice with heterozygous knockout of Acf were healthy and fertile, but they had a tissue-specific increase in endogenous Apob RNA editing activity. Homozygous deletion of Acf in mice resulted in embryonic lethality, likely due to a developmental defect in blastocyst growth leading to preimplantation failure. In support of these findings, Acf knockdown in rat hepatoma cells and primary murine hepatocytes, which express Apobec1, and in human HepG2 hepatoma cells, which do not express APOBEC1, resulted in apoptotic death. The authors concluded that ACF plays an APOBEC1-independent role in cell survival, particularly during early embryonic development.