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Purification and general properties of the DNA-binding protein (P16) from rat liver mitochondria
Abstract:
The mitochondrial DNA-binding protein P16 was isolated from rat liver mitochondrial lysates by affinity chromatography on single strand DNA agarose and separated from DNA in the preparation by alkaline CsCl isopycnic gradients. The top fraction of the gradients contained a single polypeptide species (Mr approximately equal to 15,200) based upon SDS PAGE. Digestion of single strand DNA-bound P16 with proteinase K produced a protease-insensitive, DNA-binding fragment (Mr approximately equal to 6,000) that has been purified by essentially the same procedures used for intact P16. The partial amino acid compositions for P16 and the DNA-binding fragment were obtained by conventional methods. Analysis of subcellular fractions revealed that nearly all of the cellular P16 was located in the mitochondria and that only trace amounts of protein of comparable electrophoretic mobility could be isolated from the nuclear or cytoplasmic fractions. The labeling of P16 with [35S]methionine in primary rat hepatocyte cultures was inhibited by more than 90% by the cytoplasmic translation inhibitor cycloheximide, but unaffected by the mitochondrial-specific agent chloramphenicol. These results indicate that P16 is synthesized on cytoplasmic ribosomes and imported into the mitochondria. The addition of purified P16 to deproteinized mitochondrial DNA resulted in the complete protection of the labeled nascent strands of displacement loops against branch migrational loss during cleavage of parental DNA with SstI, thus providing strong evidence that P16 is the single entity required for this in vitro function. Incubation of P16 with single strand phi X174 DNA, double strand (RF) phi X174 DNA, or Escherichia coli ribosomal RNA and subsequent analysis of the nucleic acid species for bound protein indicated a strong preference of P16 for single strand DNA and no detectable affinity for RNA or double strand DNA. Examination of P16-single strand phi X174 DNA complexes by direct electron microscopy revealed thickened, irregular fibers characteristic of protein-associated single strand DNA.
Insights
Mitochondrial DNA-binding protein P16, synthesized in the cytoplasm and imported into mitochondria, protects nascent DNA strands. This protein shows a strong preference for single-stranded DNA, crucial for mitochondrial DNA stability.
Area of Science:
- Mitochondrial Biology
- Molecular Genetics
- Protein Biochemistry
Background:
- Mitochondria possess their own DNA (mtDNA) which requires specific proteins for maintenance and replication.
- The precise roles of many mtDNA-binding proteins are not fully elucidated.
- Understanding protein interactions with mtDNA is key to comprehending mitochondrial function and dysfunction.
Purpose of the Study:
- To isolate and characterize a novel rat liver mitochondrial DNA-binding protein, designated P16.
- To determine the synthesis location and subcellular localization of P16.
- To investigate the functional role of P16 in mitochondrial DNA metabolism, specifically in protecting nascent DNA strands.
Main Methods:
- Affinity chromatography using single-stranded DNA agarose for protein isolation.
- Alkaline CsCl isopycnic density gradients for DNA separation.
- SDS-PAGE and proteinase K digestion for protein characterization and fragment analysis.
- Subcellular fractionation and radiolabeling ([35S]methionine) with specific inhibitors (cycloheximide, chloramphenicol) to determine synthesis and localization.
- In vitro assays with purified P16 and mitochondrial DNA (nascent strands, displacement loops) to assess DNA protection.
- Binding assays with various nucleic acids (single-stranded DNA, double-stranded DNA, rRNA) and electron microscopy to determine DNA binding specificity.
Main Results:
- A single polypeptide species, P16 (Mr ~15,200), was isolated and found to bind single-stranded DNA.
- A DNA-binding fragment (Mr ~6,000) was identified, indicating a specific DNA-binding domain.
- P16 is synthesized on cytoplasmic ribosomes and imported into mitochondria, with minimal presence in nuclear or cytoplasmic fractions.
- Purified P16 protected nascent DNA strands of displacement loops from degradation during SstI cleavage in vitro.
- P16 demonstrated a strong preference for single-stranded DNA, with no detectable affinity for double-stranded DNA or rRNA.
- Electron microscopy confirmed protein association with single-stranded DNA, forming thickened fibers.
Conclusions:
- P16 is a cytoplasmically synthesized, mitochondrially imported protein that binds single-stranded DNA.
- P16 plays a critical role in stabilizing mitochondrial DNA, particularly nascent strands, preventing their loss.
- The protein's specificity for single-stranded DNA suggests a role in processes involving transient single-stranded mtDNA intermediates.