Identification and characterization of a new human gene (APOC4) in the apolipoprotein E, C-I, and C-II gene locus.

We have identified and characterized a previously unreported human gene that is found within the apolipoprotein (apo) E/C-I/C-II gene locus. On the basis of its location and its properties, this new gene has been designated APOC4. Nucleotide sequence analysis of genomic DNA and liver cDNA clones revealed a 3.3-kb gene consisting of three exons and two introns. Its 3' terminus lies 555 bp upstream of APOC2, giving both genes the same transcriptional orientation. The promoter of the APOC4 gene lacks a typical TATA box, consistent with an apparent heterogeneity in transcription start sites. RNase protection analysis indicated relatively low apoC-IV mRNA levels in human liver, compared to apoC-II mRNA levels. The predicted apoC-IV protein sequence, comprising 127 amino acid residues, contains a putative 25-residue signal peptide and two potential amphipathic alpha-helical domains. Amino acid sequence comparisons indicate a limited homology between apoC-IV and either apoC-I or apoC-II. Since its hepatic expression and predicted protein structure are characteristic of the other genes in this cluster, we propose that the APOC4 gene is a member of the apolipoprotein gene family.