What proteins are synthesized by free ribosomes?

Proteins are the major building blocks of life. Eukaryotic cells synthesize proteins for thousands of different functions. Some examples:

• to build the components of the cytosol (e.g. microtubules, glycolytic enzymes);
• to build the receptors and other molecules exposed at the surface of the cell embedded in the plasma membrane;
• to supply some of the components of the mitochondria and (in plant cells) chloroplasts;
• proteins secreted from the cell to supply the needs of other cells and tissues (e.g. collagens to support cells, hormones to signal them).

All proteins are synthesized by ribosomes using the information encoded in molecules of messenger RNA (mRNA). This process is called translation and is described in Gene Translation: RNA -> Protein. Our task here is to explore the ways that these proteins are delivered to their proper destinations.

• one set for those proteins synthesized by ribosomes that remain suspended in the cytosol, and
• a second set for proteins synthesized by ribosomes that are attached to the membranes of the endoplasmic reticulum (ER) forming "rough endoplasmic reticulum" (RER). This electron micrograph (courtesy of Keith Porter) shows the RER in a bat pancreas cell. The clearer areas are the lumens.

The signal sequence in many proteins consists of the first portion of the elongating polypeptide chain (so the signal sequence occurs at the amino terminal of the polypeptide). Typical signal sequences contain about 15 amino acids. The precise amino acid sequence varies surprisingly from one protein to the next, but all signal sequences include many hydrophobic amino acids.

Ribosomes are small structures within biological cells that function in protein synthesis. Proteins are essential for a huge range of activities within an organism, from catalyzing metabolic reactions and protecting against infection to providing shape and structure. Protein synthesis has several different stages, and ribosomes, which may be free or membrane-bound, are an essential part of the pathway.

Ribosome Structure and RNA

Ribosomes consist of two basic components, a large component and a small component, made up of RNA molecules. RNA is similar to DNA but is single-stranded and shorter in length with slightly different chemical components. DNA enables the first stage in protein synthesis -- transcription -- which takes place inside the cell nucleus. This produces a type of RNA known as messenger RNA, or mRNA. Ribosomes then convert mRNA into protein, in a process known as translation.

Codons and Protein Synthesis

The mRNA carries the genetic information encoded in the original DNA, acting as a messenger to produce the appropriate proteins, hence the name. It is arranged into areas called codons, each consisting of three small molecules called nucleotides. Each codon specifies a particular amino acid, the building blocks of proteins. Another type of RNA known as transfer RNA, or tRNA, reads the nucleotides. In this way, amino acids are produced and protein chains built up.

Free and Membrane-Bound Ribosomes

Ribosomes are found in two locations in the cell. Free ribosomes are present in the cytosol, the watery fluid inside the cell and are not attached to any other structure. Membrane-bound ribosomes are attached to a structure known as rough endoplasmic reticulum. Free and membrane-bound ribosomes produce different proteins. Whereas membrane-bound ribosomes produce proteins that are exported from the cell to be used elsewhere, free ribosomes produce proteins used inside the cell itself.

Proteins Produced by Free Ribosomes

Ribosomes are important because they are responsible for protein synthesis. Free ribosomes, in particular, are important because they produce proteins essential for internal cellular activity, which are not synthesized elsewhere. These include proteins used in the cytosol and the supportive structure known as the cytoskeleton, as well as those used by the mitochondria, which produces the cell’s energy, and, in plant cells, chloroplasts. Without free ribosomes, the various components of the cell could not function.

References

• Scitable by Nature Education: Ribosomes, Transcription, and Translation
• Scitable by Nature Education: Translation: DNA to mRNA to Protein

Writer Bio

Based in Scotland, Clare Smith is a writer specializing in natural science topics. She holds a Master of Science in plant biodiversity from the University of Edinburgh.

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