Casein composition and differential translational efficiency of casein transcripts in donkey's milk

Area of Science:

• Biochemistry
• Molecular Biology
• Animal Science

Background:

• Donkey milk is a valuable source of nutrients.
• Caseins are the primary proteins in milk, crucial for nutrition and cheese production.
• Understanding casein composition and synthesis in donkey milk is important for its utilization.

Purpose of the Study:

• To quantify the four major casein proteins (αs1, αs2, β, and κ-CN) in donkey milk.
• To determine the mRNA levels of corresponding casein genes.
• To investigate the relationship between casein gene expression and protein abundance, exploring translational efficiency.

Main Methods:

• Urea-Polyacrylamide Gel Electrophoresis (Urea-PAGE) for casein protein separation and quantification.
• Immuno-detection with polyclonal antibodies and densitometric analysis for precise protein measurement.
• Reverse Transcription Quantitative Polymerase Chain Reaction (RT-qPCR) for mRNA quantification of casein transcripts.
• Calculation of translation efficiency by comparing protein fraction percentages to transcript percentages.

Main Results:

• Beta-casein (β) was the most abundant protein (54.28%), followed by αs1-casein (35.59%), αs2-casein (7.19%), and κ-casein (κ-CN) (2.79%).
• The relative abundance of casein gene transcripts was CSN2 (70.85%), CSN1S2 (14.23%), CSN3 (8.65%), and CSN1S1 (6.28%).
• Significant variations in translation efficiency were observed, ranging from 0.32 (κ-CN) to 5.66 (αs1-casein), indicating differential protein synthesis rates.

Conclusions:

• Donkey milk casein composition is dominated by β-casein and αs1-casein.
• Discrepancies between mRNA levels and protein abundance suggest complex post-transcriptional and translational regulation.
• Sequence features, including flanking regions of the start codon, codon usage, and coding sequence length, likely influence the differential expression rates of donkey casein genes.