Human acyl-coenzyme A:cholesterol acyltransferase 1 (acat1) sequences located in two different chromosomes (7 and 1) are required to produce a novel ACAT1 isoenzyme with additional sequence at the N terminus.
A rare form of human ACAT1 mRNA, containing the optional long 5'-untranslated region, is produced as a 4.3-kelonucleotide chimeric mRNA through a novel interchromosomal trans-splicing of two discontinuous RNAs transcribed from chromosomes 1 and 7. To investigate its function, we express the chimeric ACAT1 mRNA in Chinese hamster ovary cells and show that it can produce a larger ACAT1 protein, with an apparent molecular mass of 56 kDa on SDS-PAGE, in addition to the normal, 50-kDa ACAT1 protein, which is produced from the ACAT1 mRNAs without the optional long 5'-untranslated repeat. To produce the 56-kDa ACAT1, acat1 sequences located at both chromosomes 7 and 1 are required. The 56-kDa ACAT1 can be recognized by specific antibodies prepared against the predicted additional amino acid sequence located upstream of the N-terminal of the ACAT1(ORF). The translation initiation codon for the 56-kDa protein is GGC, which encodes for glycine, as deduced by mutation analysis and mass spectrometry. Similar to the 50-kDa protein, when expressed alone, the 56-kDa ACAT1 is located in the endoplasmic reticulum and is enzymatically active. The 56-kDa ACAT1 is present in native human cells, including human monocyte-derived macrophages. Our current results show that the function of the chimeric ACAT1 mRNA is to increase the ACAT enzyme diversity by producing a novel isoenzyme. To our knowledge, our result provides the first mammalian example that a trans-spliced mRNA produces a functional protein.
State Key Laboratory of Molecular Biology, Institute of Biochemistry and Cell Biology, Shanghai Institutes for Biological Sciences, Chinese Academy of Sciences, 320 Yue-Yang Rd., Shanghai 200031, China.
Li
L
Yang
Oneil
O
Lee
Jia
J
Chen
Jiang
J
Chen
Catherine C Y
CC
Chang
Pei
P
Zhou
Zhen-Zhen
ZZ
Wang
Han-Hui
HH
Ma
Hui-Fang
HF
Sha
Jiu-Xian
JX
Feng
Yi
Y
Wang
Xin-Ying
XY
Yang
Li
L
Wang
Ruhong
R
Dong
Kim
K
Ornvold
Bo-Liang
BL
Li
Ta-Yuan
TY
Chang
D000595:Amino Acid Sequence; D002878:Chromosomes, Human, Pair 1; D002897:Chromosomes, Human, Pair 7; D004721:Endoplasmic Reticulum / Q000201:enzymology; D005091:Exons; D006801:Humans; D007527:Isoenzymes / Q000737:chemistry / Q000235:genetics*; D008264:Macrophages / Q000201:enzymology; D008969:Molecular Sequence Data; D008970:Molecular Weight; D014176:Protein Biosynthesis; D002785:Sterol O-Acyltransferase / Q000737:chemistry / Q000235:genetics*
D007527:Isoenzymes; D002785:Sterol O-Acyltransferase