A sequence-specific RNA-binding protein complements apobec-1 To edit apolipoprotein B mRNA

Area of Science:

• Biochemistry
• Molecular Biology
• RNA Metabolism

Background:

• Apolipoprotein B (apo-B) mRNA editing is a crucial post-transcriptional modification.
• This process involves site-specific deamination of cytidine to uracil, directed by a downstream mooring sequence.
• The catalytic subunit, Apobec-1, requires accessory proteins (complementing activity) for efficient in vitro editing, but their function remains unclear.

Purpose of the Study:

• To identify and characterize the protein(s) responsible for the complementing activity in apo-B mRNA editing.
• To elucidate the role of these accessory factors in the editing complex.

Main Methods:

• RNA affinity chromatography using wild-type and mutant apo-B RNA.
• UV cross-linking assays to detect RNA-protein interactions.
• Far-Western analysis to assess protein-protein interactions.
• Introduction of the mooring sequence into a heterologous RNA (luciferase) to test binding specificity.

Main Results:

• A 65-kDa protein was identified as the primary component of the complementing activity.
• This 65-kDa protein specifically binds to the apo-B mRNA mooring sequence.
• The protein was shown to UV cross-link to wild-type apo-B mRNA but not to mutant RNA lacking the functional mooring sequence.
• The 65-kDa protein also interacted with Apobec-1.

Conclusions:

• The complementing activity in apo-B mRNA editing is mediated by a 65-kDa protein.
• This protein likely functions as the RNA-binding subunit of the editing holoenzyme.
• The mooring sequence is critical for the specific recruitment of this subunit to the target mRNA.