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Biosynthesis of apolipoprotein C-III in rat liver and small intestinal mucosa
Abstract:
We have examined the biosynthesis of rat apolipoprotein C-III in the small intestine and liver. The primary translation product of its mRNA was recovered from wheat germ and ascites cell-free systems. Comparison of its NH2-terminal sequence with the NH2 terminus of plasma high density lipoprotein-associated apolipoprotein C-III showed that apo-C-III was initially synthesized as a preprotein with a 20 amino acid long NH2-terminal extension: Met-X-X-X-Met-Leu-Leu-X-X-Ala-Leu-X-Ala-Leu-Leu-Ala-X-Ala-X-Ala. Co-translational cleavage of the cell-free translation product by signal peptidase generated a polypeptide with the same NH2 terminus as the mature protein (X-Glu-X-Glu-Gly-Ser-Leu-Leu-Leu-Gly-Ser-Met). Therefore, this apolipoprotein does not undergo post-translational proteolytic processing like two other high density lipoprotein-affiliated proteins, proapo-A-I and proapo-A-II. The mRNA encoding apolipoprotein C-III comprises 0.4% of the translatable RNA species in adult rat liver and 0.14% of the translatable RNA species in small intestinal epithelium. Acute fat feeding with a triglyceride meal resulted in a 2-fold increase in intestinal preapo-C-III mRNA accumulation but no change in the levels of preproapo-A-I mRNA. Thus, the acute response of the apo-A-I and C-III genes to triacylglycerol absorption differs.
Insights
Rat apolipoprotein C-III (apo-C-III) is synthesized as a preprotein in the small intestine and liver. Fat feeding increases intestinal apo-C-III mRNA, unlike apo-A-I mRNA, indicating differential gene regulation.
Area of Science:
- Biochemistry
- Molecular Biology
- Genetics
Background:
- Apolipoprotein C-III (apo-C-III) is a component of high-density lipoproteins (HDL).
- Understanding apo-C-III biosynthesis is crucial for lipid metabolism research.
Purpose of the Study:
- To investigate the biosynthesis of rat apo-C-III in the small intestine and liver.
- To determine if apo-C-III undergoes post-translational processing.
- To examine the regulation of apo-C-III gene expression in response to dietary fat.
Main Methods:
- Cell-free translation systems (wheat germ, ascites) were used to identify the primary translation product of apo-C-III mRNA.
- NH2-terminal sequencing was performed on the cell-free translation product and plasma apo-C-III.
- Quantification of apo-C-III and apo-A-I mRNA levels in liver and small intestine was done using Northern blot analysis.
- Effects of acute fat feeding on mRNA levels were assessed.
Main Results:
- Apolipoprotein C-III is initially synthesized as a preprotein with a 20-amino acid N-terminal signal sequence.
- Signal peptidase mediates co-translational cleavage, yielding mature apo-C-III without further proteolytic processing.
- Apo-C-III mRNA constitutes 0.4% of translatable RNA in adult rat liver and 0.14% in the small intestine.
- Acute triglyceride feeding doubled intestinal preapo-C-III mRNA but did not affect preproapo-A-I mRNA levels.
Conclusions:
- Rat apo-C-III biosynthesis involves co-translational cleavage of a preprotein, not post-translational processing.
- The expression of apo-C-III and apo-A-I genes responds differently to acute dietary fat intake.
- These findings highlight distinct regulatory mechanisms for apo-C-III and apo-A-I in lipid absorption.