Are Ribosomes Proteins? 5 Facts You Should Know


Yes, ribosomes do contain proteins. Ribosomes function as micro-machine for synthesizing proteins. These contain rRNA and ribosomal proteins. It moves along the mRNA and catalyzes the process of translation.

Some interesting facts –

Ribosomes can be classified either as free or membrane bound

These ribosomes differ based on spatial distribution. Some exist free in the cytoplasm while others are embedded on endoplasmic reticulum.

Free ribosomes can move everywhere in the cytosol and synthesize proteins that are utilized by the cell. However membrane bound ribosomes usually produce proteins necessary for plasma membrane and are transported in secretory vesicles.

Proteins synthesized by cytosolic ribosomes

Free ribosomesMembrane -bound ribosome
Soluble cytosolic protein
Peroxisomal proteins
Mitochondrial and chloroplast proteins encoded by
nuclear DNA
Secretory protein
Integral plasma membrane protein
Lysosomal protein
Endoplasmic reticulum protein

Why are ribosomes referred to as protein factories?

Ribosomes are unique machinery that can link 200 amino acid residues per minute. It requires two to three hours to produce massive proteins of about 30,000 amino acid residues.

Ribosomes are the main large ribonucleoproteins, ubiquitously present in the cell continuously involved in the synthesis of protein. They can move along the mRNA in the direction 5’ to 3’, linking the amino acids into a polypeptide chain. Thus, these organelles are referred to as protein factories.

A ribosome consists of two subunits – one larger subunit and one smaller subunit along with rRNA and a small number of proteins. The most common cytosolic ribosome is the 80S (eukaryotic ribosome) and the prokaryote member is 70S. The two subunits lock together at a specific site on the mRNA called the Shine-Dalgarno sequence. After the association, it initiates the translation of the encoded information as provided by the mRNA.

are ribosomes proteins
Image credit – Wikipedia
Structure and shape of the E.coli 70S ribosome. Larger subunit in red while smaller subunit in blue
Prokaryotic ribosomeEukaryotic ribosome
Larger subunit50S ribosomal subunit; 23S and 5S rRNA; 31 proteins60S ribosomal subunit; 28S rRNA, 5.8S rRNA and 5S rRNA; 49 proteins
Smaller subunit30S ribosomal subunit; 16S rRNA; 21 proteins40S ribosomal subunit; 18S rRNA; 33 proteins

The larger subunit has the catalytic center and performs the role of an enzyme called ribozyme. While the smaller one mainly functions by decoding the information present in the codon and linking the amino acids.

Are ribosomes bigger than proteins?

Not all proteins are smaller than the ribosomes.

Yes, ribosomes are larger than most of the proteins. Prokaryotic ribosomes are smaller than eukaryotic ones. The ribosomes are spherical in shape. The size of prokaryotic ribosomes is 20 nm and eukaryotic ribosome size ranges from 25 to 30 nm.  In comparison, the size of the common protein Rubisco is 3-6 nm in diameter.

ProteinDimension (nm)
Catalase9.7 ×9.2× 6.7
Phosphofructokinase14 ×9 ×9
Serum albumin7.5 ×6. ×5 4
Hemoglobin6× 5× 5
Ovalbumin7× 3.6 × 3

However, there are proteins larger than ribosomes. Those proteins contain multimers that are separately synthesized and through post-translational modifications, they transform into globular proteins.  

How are ribosomes and proteins related?

Are ribosomes proteins? Well it is an interesting question. Ribosomes and proteins are complementary to each other. Without ribosomes proteins will not be present.

Without ribosomes, proteins cannot be synthesized. Ribosomes are an integral component in protein synthesis. The translation of proteins takes place in three highly regulated steps.

Initiation

In this stage, the smaller unit of ribosome recognizes and gets linked to the start site present on the mRNA. Then tRNA enters the smaller subunit. Soon after larger ribosomal subunit joins and completes the complex. Charged tRNA enters the site A of the ribosome and initiates protein synthesis.

Elongation

This process comprises the phase between initiation and termination. The charged tRNA placed in site A is moved to site P and another newly charged tRNA comes into site A.

Every charged tRNA donates an amino acid which gets attached to the growing polypeptide chain through a peptide bond synthesized by a peptidyl transferase. this reaction takes place in the larger subunit of the ribosome. The process of movement of charged tRNAs from site A to site P along with the ribosome is called translocation.

When the nascent amino acid is added to the polypeptide chain, the tRNA is moved to site E and exits the ribosomal complex. This way cycle of events continues until the termination stage arrives. 

are ribosomes proteins
Image credit – Wikimedia
Ribosome during translation process

Termination

There are specific sites on the mRNA which recruit some factors called release factors that prevent any other tRNA to bind the ribosomal sites. The newly formed polypeptide chain leaves the ribosomal complex and the two subunits of the ribosome disintegrate and get separated.

Are ribosomal proteins produced by translation?

After the translation, the nascent ribosomal proteins are imported back to the nucleus and then to the nucleolus and get associated with ribosomal RNA to form pre-ribosomes. These pre-ribosomes complexes undergo various conformational changes and are exported back to the cytoplasm to perform translation functions.

Yes, ribosomal proteins are produced by translation. Each mRNA encoding for ribosomal protein is first transported to the cytoplasm where translation takes place. In yeast, approximately 150,000 ribosomal proteins cross the nuclear envelope per minute.

Conclusion

Ribosomes are larger ribonucleoproteins, play indispensable role in protein synthesis. It has two subunits, one larger subunit and one smaller subunit, both with unique functionalities. It also contain rRNA and ribosomal proteins. In nucleus ribosome biogenesis takes place. However ribosomal proteins are translated in cytoplasm and imported back to the nucleus for association with rRNA.

Paurabi Das

I am a doctoral student of CSIR- CIMAP, Lucknow. I am devoted to the field of plant metabolomics and environmental science. I have completed my post-graduation from the University of Calcutta with expertise in Molecular Plant Biology and Nanotechnology. I am an ardent reader and incessantly developing concepts in every niche of biological sciences. I have published research articles in peer-reviewed journals of Elsevier and Springer. Apart from academic interests, I am also passionate about creative things such as photography and learning new languages. Let’s connect over Linkedin- https://www.linkedin.com/in/paurabi-das-cimap26/

Recent Posts