Ribosomes

Key features Eukaryotic ribosomes

• Most abundant RNA-protein complex in a cell

• Typical cell contains millions of ribosomes

• Some are found free in the cytoplasm, others associate with endoplasmic reticulum

• Has the molecular machinery for reading and translating genetic code of mRNA and mediate protein synthesis

synthesis of polypeptide chains is at a rate of

2-3 amino acids per second! Small proteins (100 – 200 amino acids) made in minutes. Large proteins (e.g. TTN, Titin; 30, 000 amino acids);2-3 hours

Differences in efficiency of translation

can significantly impact protein synthesis.

Ribosomes are composed of

a large (60S) and small (40S) subunit, They assemble during translation to form (80S) ribosome

indirect measure of particle size, density and shape

S = Svedberg units = sedimentation rate in an ultracentrifuge

Each subunit composed of rRNA and ribosomal proteins

Large (60S) subunit : 47 ribosomal proteins, 28S, 5.8S and 5S rRNA

Small (40S) subunit : 33 ribosomal proteins, 18S rRNA

Ribosome biogenesis

1. pre-rRNA is encoded from rDNA

2. 40S and 60S are then processed with respective subcomplexes (over 30 non ribosomal protein factors) and factors (about 80)

3. they join with respective ribosomal proteins and move from the nucleolus to the cytoplasm

pre-ribosomes interact with…

…export receptors to move through the hydrophobic central channel of the nuclear pore complex

Ribosome heterogeneity

Ribosomes are compositionally heterogeneous between species and even within the same cell, as evidenced by the existence of cytoplasmic and mitochondria ribosomes within the same eukaryotic cells.

ribosomal heterogeneity in mammals is suggested to be important for

gene regulation, i.e., the specialized ribosome hypothesis. However, this hypothesis is controversial and the topic of ongoing research