Translation and Protein Synthesis

  • Translation refers to the process of converting messenger RNA (mRNA) into proteins.

  • Occurs within the cytosol of the cell.

  • A single mRNA molecule has the capacity to generate multiple proteins.

Components of the Translation Machinery

  • Translation involves three primary components:

    1. Messenger RNA Template (mRNA)

    2. Ribosome

    3. Transfer RNA (tRNA)

RNA Processing

  • Primary RNA transcript contains exons and introns.

  • Example Structure:

    • Exon 1 Intron Exon 2 Intron Exon 3

  • After splicing, the RNA is processed into:

    • Exon 1 Exon 2 Exon 3

  • Additional modifications include:

    • 5' Cap: Protects the mRNA from degradation and assists in ribosome binding.

    • Poly-A Tail: Enhances mRNA stability and export from the nucleus.

    • 5' Untranslated Region (UTR): Region before the start codon.

    • 3' Untranslated Region (UTR): Region following the stop codon.

The Ribosome

  • The ribosome serves the function of translating mRNA into proteins.

  • Structure:

    • Composed of ribosomal RNA (rRNA) and associated proteins.

    • Exists in two forms: free in the cytosol or attached to the endoplasmic reticulum (ER). - Ribosomes attached to the ER are referred to as 'rough ER'.

Eukaryotic Ribosome Specifics

  • Ribosome subunit composition:

    • Large Subunit (60S): Composed of 28S, 5.8S, and 5S rRNA.

    • Total rRNA nucleotide counts:

    • 28S (4800 rNTs), 5.8S (160 rNTs), 5S (120 rNTs).

    • Small Subunit (40S): Contains 18S rRNA (1900 rNTs).

    • Total: 80S ribosome in eukaryotes.

Transfer RNA (tRNA)

  • Function: Specific tRNAs exist for each amino acid.

  • Each tRNA molecule is linked to its corresponding amino acid.

  • tRNA Structure:

    • Contains a triplet of nucleotides known as an anticodon, which pairs with the corresponding codon on mRNA.

    • Base-pairing facilitates the positioning of the correct amino acid for incorporation into the growing polypeptide chain.

Amino Acid Activation and tRNA Role

  • Example: Activation of the amino acid Phenylalanine (Phe).

    • The structure of phenylalanine:

    • ext{H}_2 ext{N}- ext{C}- ext{C}- ext{O} ext{H}

    • Reaction:

    • Attachment of Phe to tRNA requires ATP:

    • ext{ATP} + ext{tRNA}{ ext{Phe}} ightarrow ext{AMP} + ext{PPi} + ext{tRNA}{ ext{Phe}}

    • High-energy ester bond is formed, allowing tRNA_Phe to bind to the UUU codon on mRNA.

tRNA Structure

  • tRNA exhibits a highly structured form, typically represented as three loops and one stem, folding into an 'L' shape in three dimensions.

Anticodon and Codon Interactions

  • The interaction between the tRNA anticodon and mRNA codon is critical for correct polypeptide synthesis.

  • Example of an anticodon-codon pairing:

    • tRNA Anticodon: UAG

    • mRNA Codon: AUC

Polypeptide Chain Elongation

  • As translation proceeds, the ribosome elongates the growing polypeptide chain by adding new amino acids correspondingly.

  • Multiple polypeptide chains can be synthesized concurrently from the same mRNA strand.

  • Protein Sorting: Begins even before the polypeptide chain exits the ribosome.

    1. Pathway A: Cytoplasmic

    2. Pathway B: Endomembrane System

Outcomes Based on Ribosome Location

  • Free ribosomes in the cytosol synthesize proteins with different destinations than those synthesized by ribosomes on the ER.

  • Destinations of proteins synthesized by free ribosomes include:

    1. Cytosol (no peptide signal required)

    2. Organelles (e.g., nucleus, mitochondria, or chloroplasts, requiring targeting peptide signals)

  • Proteins synthesized by ER-associated ribosomes are directed toward the endomembrane system.

Ribosome-Associated Translation

  • Initiation of translation can either occur in the cytosol or at the rough ER membrane.

  • Following synthesis, proteins can either remain in the cytosol or be transported to other organelles such as lysosomes or mitochondria.

Endomembrane System Transport

  • Cotranslational Import: The mechanism facilitating the transport of proteins to the ER during translation.

  • N-terminal signal peptides guide the ribosome and polypeptide to the ER prior to full translation.

  • This system encompasses the ER, Golgi complex, and vesicles involved in transport.

Protein Import to Mitochondria

  • Import involves ATP-powered translocation through the mitochondrial membranes utilizing receptors (like Tim23/17 and Tom40) for proper localization.

Transport to the Golgi Complex

  • Proteins and lipids travel from the ER to Golgi complex through small membrane-bound vesicles, known as anterograde transport.

  • Reverse transport (from Golgi back to ER) is termed retrograde transport, crucial for recycling ER materials.

Types of Vesicle Coat Proteins

  • Vesicle types based on coat proteins:

    1. Clathrin

    2. COPI

    3. COPII

  • Functions of these coat proteins include:

    • Targeting, vesicle budding, fusion prevention, and interaction with microtubules.

Anatomy of the Golgi Complex

  • The Golgi complex consists of flattened membrane-bound discs termed cisternae.

  • Divided into three sections:

    1. Cis-Golgi Network (CGN): Nearest to the ER.

    2. Medial Cisternae

    3. Trans-Golgi Complex (TGN): Faces away from the ER.

Protein Glycosylation in the Golgi

  • Proteins undergo glycosylation, a key post-translational modification, where carbohydrates are added to specific amino acid residues (serine, threonine, asparagine).

  • While initiated in the ER, glycosylation is finalized in the Golgi.

Lysosomal Transport of Proteins

  • Certain proteins are designated for lysosomal use and tagged with mannose 6-phosphate before transport from the Golgi to late endosomes, converting to lysosomes.