Do Lysosomes Have Proteins? 7 Facts (What, Where, How)

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Lysosomes are membrane bound organelles that possess several types of enzymes. They are involved in the degradation of several biomolecules. Let us see if it has proteins.

Lysosomes have proteins within its structure as it forms a major part of biomolecules that get broken down in the lysosomes. These proteins are usually misfolded proteins and cyclic proteins or are present as structural components.

Let us discuss the types of proteins found in lysosomes, how proteins are produced in lysosomes, its functions and some other related topics in this article.

What are the proteins found in lysosomes?

Lysosomal proteins can be present in the form of membrane proteins, as the enzymes that bring about catalytic reactions and as those that get broken down. Some types of lysosomal proteins are listed below:

  • Proton pumps– Lysosomal enzymes can function only at a low pH which is maintained by the proton pumps. A low pH keeps the organelle from digesting the contents of a cell, accidentally. In case of accidental release of the lysosomal contents, its enzymes become inactive in the cytosol as it has neutral pH.
  • LAMPs- LAMPs or lysosome associated membrane proteins are a part of the lysosomal membrane. They are glycoproteins. They take part in the transport of hydrolase enzymes and lipids, Phagocytosis and autophagy.
  • Enzymes- Lysosomes are rich in different types of enzymes which are basically proteins. The precursors of these enzymes are biosynthesized in the endoplasmic reticulum which is followed by the modifications and packaging in the Golgi apparatus.
  • Misfolded proteins- Different environmental factors and stochastic factors may lead to misfolding of proteins. Accumulation of such misfolded proteins in the cell can cause disorders.
  • Cyclic proteins- Some regulatory proteins undergo periodic controlled degradation. An example is the cyclin proteins that are present at the checkpoints during the process of cell division.

Where are lysosome proteins produced?

Lysosomes bring about protein degradation to produce amino acids. Let us see where proteins are produced inside the lysosomes.

Proteins are degraded and recycled in the lumen of the lysosome. The lumen of the lysosomes is rich in all sorts of enzymes. Hence, these enzymatic reactions are restricted to the lumen of the organelle. The amino acids produced as a result are then transported to the cytosol.

How are lysosome proteins produced?

Lysosomes can degrade a variety of proteins. Let us discuss the steps involved in the process of degradation of misfolded proteins and production of the amino acids.

  1. Recognition of misfolded proteins: In some instances, proteins containing a specific sequence of amino acids (lysine, phenylalanine, glutamic acid, arginine and glutamine) are targeted by the lysosome for degradation.
  2. Transport of misfolded proteins: The misfolded proteins are transported to the lysosomes by either autophagy or endocytosis.
  3. Protein degradation: Cathepsins are responsible for the break down of the proteins. Cathepsins are a group of lysosomal proteins that are active at a low pH. They form the majority of the protease enzymes present in the lysosomes.

How do proteins go to lysosomes?

Transport of proteins to the lysosomes can be mediated through membrane enclosed structures which possess different recognition factors. Let us see in detail.

Autophagy and endocytosis are the ways in which cytoplasmic proteins get transferred to the lysosomes. Autophagy can be of three types- CMA (chaperone mediated autophagy), marcoautophagy and microautophagy.

1. Chaperone mediated autophagy

Target proteins have a specific sequence of amino acids which is lysine, phenylalanine, glutamic acid, arginine and glutamine. This sequence gets exposed upon misfolding and is targeted for degradation. LAMPs (or lysosome associated membrane proteins) act as the receptors in this pathway.

2. Macroautophagy

In this pathway, double membranous structures called autophagosomes encapsulate cytosolic proteins. Let us see what steps are involved in it.

  • Omegasome: These lipid bilayer structures form during the initiation of the process. They are elongated in shape.
  • Phagophore: Omegasomes elongate and form a cup shaped structure called phagophores. These structures further elongate to encapsulate the cargo such as misfolded protein.
  • Autophagosome: When the proteins get encapsulated within the membrane, they form autophagosomes which separate the cargo from the cytoplasm.

3. Microautophagy

Microautophagy is a another type of autophagy which occurs in either lysosomes or vacuoles. This pathway involves intracellular non selective degradation of protein and cellular organelles. This pathway is mainly responsible for maintaining membrane homeostasis, size of the organelle and survival of the cell under nitrogen scarcity.

What does the lysosome do to proteins?

Proteins can be present in conjugation with other biomolecules too. More than two enzymes can be needed for degradation. Let us discuss the various enzymes involved in it.

  • Cathepsin proteases: Peptidase enzymes belonging to the family of cathepsins, break down proteins and low density lipoproteins. They also mediated endocytosis and macroautophagy of several enzymes. They can take part in signaling processes too.
  • Phospholipases: They breakdown the lipoproteins that are majorly present in the cellular and organellar membranes.
  • Glycosidases: Glycogen containing compounds such as glycoproteins, glycosaminoglycans and glycosphingolipids, are usually broken down with the help of glycosidases enzymes. These cleave the carbohydrate residues present at the termini of these polymers.
  • Lysosomal acid lipases: These play critical role in membrane integrity, signaling and energy metabolism. They mediate the degradation of lipoproteins and triglycerides to free cholesterol and fatty acids respectively.
  • Sulfatases: These evolutionarily conserved family of enzymes are responsible for the break down of glycosaminoglycans. Some examples of glycosaminoglycans are heparin, heparan sulphate, keratan sulphate, sulfolipids and sulphur container hormones.
  • Nucleases: These enzymes mediate the hydrolysis of nucleic acids such as RNAs (Ribonucleic acids) and DNAs (deoxyribonucleic acids).

Function of proteins in lysosomes?

Lysosome contains a huge variety of proteins, each having varied structure and even more varied functions. Let us discuss some of their functions.

  • Membrane proteins: Proteins present in the membrane regulate the entry and exit of materials to and from the lysosome. They also regulate the fusion of phagosomes with the membrane.
  • Proton pumps: These are embedded in the membrane of the lysosomes and allow it to have an acidic pH by pumping in hydrogen ions. The highly acidic environment of the organelle allows the enzymes to get activated.
  • Enzymes: Lysosomes are a store house of different enzymes. They are responsible for the degradation of different macromolecules into their respective constituents.

Does lysosome make proteins?

Lysosomal enzymes breakdown the proteins either selectively or non selectively in order
to produce amino acids which can be reused by the cell.

Lysosomes do not produce proteins but their building blocks, that is, amino acids does. Usually, cell breakdown all proteins that are brought to it and is not required by the cell anymore. But, when cells are starved, they carry out selective degradation of proteins in order to provide the cell with amino acids.

Conclusion

Breakdown of proteins is not only important for recycling them but also to prevent them from accumulating in the cells. Studies have suggested that the capacity to breakdown the
proteins get reduced with age.

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