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Ferritin mRNA is found on bound as well as on free polyribosomes in rat heart
C H Campbell1, R Ismail, M C Linder
1Department of Chemistry and Biochemistry, California State University, Fullerton 92634.
Abstract:
Free and endoplasmic reticulum-bound polyribosomes from rat heart were examined for their ferritin mRNA content. A procedure for separation and purification of the two ribosome populations that produced good yields of homogeneous mono- and polyribosomes with no contaminating ultrastructures and gave distinctive sedimentation profiles in 15-50% sucrose gradients was developed. 14C-labeled free and bound polyribosomes added to heart preparations indicated that only 3% of free and 5.5% of bound polyribosomes cross-contaminated the bound and free fractions, respectively. RNA from both polyribosome populations hybridized with [32P]cDNA for rat ferritin. The extent of hybridization with mRNA from endoplasmic reticulum (ER)-derived polyribosomes was much greater than what could be accounted for by cross-contamination with free polyribosomes. This indicates that heart ferritin is synthesized not only on free polyribosomes for internal use in iron storage but also on ER-bound polyribosomes, where it may be destined for secretion into the plasma.
Insights
Rat heart cells synthesize ferritin, a key protein for iron storage, on both free and endoplasmic reticulum-bound ribosomes. This suggests ferritin may be secreted into the plasma in addition to intracellular iron storage.
Area of Science:
- Molecular Biology
- Cell Biology
- Biochemistry
Background:
- Ferritin is the primary intracellular iron-storage protein.
- The synthesis location of ferritin in cardiac tissue was previously unclear.
Purpose of the Study:
- To investigate the cellular localization of ferritin synthesis in rat heart.
- To determine if ferritin is synthesized on free or endoplasmic reticulum-bound polyribosomes.
Main Methods:
- Developed a method to isolate and purify free and endoplasmic reticulum-bound polyribosomes from rat heart.
- Utilized sucrose gradient centrifugation for ribosome separation.
- Quantified ferritin mRNA levels using hybridization with complementary DNA (cDNA).
Main Results:
- Successfully isolated pure populations of free and bound polyribosomes with minimal cross-contamination.
- Detected ferritin mRNA in both free and endoplasmic reticulum-bound polyribosome fractions.
- Observed significantly higher ferritin mRNA levels in the endoplasmic reticulum-bound fraction than could be explained by contamination.
Conclusions:
- Ferritin is synthesized on both free polyribosomes for intracellular iron storage and on endoplasmic reticulum-bound polyribosomes in rat heart.
- Synthesis on endoplasmic reticulum-bound polyribosomes suggests a potential role for ferritin secretion into the plasma.