Post translational Modifications of Proteins

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  • 0:03 Post-Translational Modifiation
  • 1:51 Phosphorylation
  • 3:00 Ubiquitylation
  • 4:10 Other Modifications
  • 5:28 Lesson Summary
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Lesson Transcript
Instructor: Kristin Klucevsek

Kristin has taught college Biology courses and has her doctorate in Biology.

Just because a polypeptide has been translated from an mRNA sequence, doesn't mean the protein is complete. In this lesson, you'll learn about possible post-translational modifications, including phosphorylation, ubiquitylation, glycosylation, and cleavage.

Post-Translational Modifications

Ok, so you took a shower and got dressed today in a simple pair of pants and a t-shirt. That's a good start. In some cases, that's plenty. That's good enough. But sometimes you're going to need that little something extra. If you're playing soccer today, for example, you're going to need that pair of cleats and a cool headband. If you're playing in the snow, a scarf, gloves, and fleece hat are kind of essential. If you were to, let's say, do something completely extraordinary and go scuba diving today, well, you're going to need a wetsuit. And an oxygen tank.

Let's call these 'extras' post-dressing modifications. They are additional items you'd need to do something specific. Now let's liken these to post-translational modifications. I assure you, they are just as interesting as the something extra that you'd consider putting on to have fun today. They too specify certain activities or roles, only for proteins. Post-translational modifications are changes made to a polypeptide or protein any time after translation. Now, 'Whoa, wait a minute!' you say - after a polypeptide is made from RNA during translation, aka protein synthesis, it's not complete? It needs something extra? Well, sometimes you're fine in a t-shirt and pants. Sometimes, it's not quite enough.

Post-translational modifications can include cleaving a polypeptide or adding something to it. In this lesson, we'll talk about some of these modifications and why they help specify or change a protein's function, much like how the finishing touches on your outfit might have a lot to do with what you're going to do today. Post-translational modifications occur on both eukaryotic and prokaryotic proteins. However, not all the same ones occur in both cells. In this lesson, we'll focus on the post-translational modifications that can happen to proteins specifically in eukaryotic cells.


Phosphorylation is a type of post-translational modification that adds a phosphate group to a protein. This is often added to a serine, tyrosine, or threonine amino acid residue. A kinase is the protein enzyme that adds this phosphate group. Not all proteins are phosphorylated. However, for some proteins, phosphorylation can activate or even sometimes inactivate a protein. You heard that right - when a kinase phosphorylates a target protein, it can turn the protein 'on'. However, there are some proteins that are actually turned 'off' by phosphorylation by a kinase. It depends on the protein that is being phosphorylated.

Now, just as you might put on a pair of cleats to play soccer, you should definitely take them off when you're off the field. You wouldn't want to walk through the house in those spikes. Similarly, phosphorylation can be reversed. Dephosphorylation is the removal of a phosphate group. A phosphatase is a protein enzyme that removes a phosphate group. Dephosphorylation would do the opposite of whatever phosphorylation did. So, if adding a phosphate activated a protein so that it could do a specific job, then dephosphorylation would deactivate a protein so that it would stop doing that job, and vice versa.


Ubiquitylation, also called ubiquitination, is a type of post-translational control that adds ubiquitin to a protein. A ubiquitin is a small regulatory protein that can have different functions. Ubiquitins are added to proteins by a specialized cascade of enzymes.

Often, when a protein is modified with a chain of ubiquitin proteins, that ubiquitylated protein is targeted for degradation or destruction. It's like putting a tag on a protein to signal that it's time to take this protein out to the trash. This would happen when a protein is no longer needed.

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