I am not a biologist, and I have a probably dumb biological question. For some purpose, I need to understand the function of the CTNS gene, and here is the definition of it:

"This gene encodes a seven-transmembrane domain protein that functions to transport cystine out of lysosomes.

I did not know what cystine is, and when I checked Wikipedia, I see that "Cystine is the oxidized dimer form of the amino acid cysteine."

What I understand is lysosomes digest a bigger peptide, and cystine is created as a result, and it is taken out of lysosomes by the enzyme produced thanks to the gene CTNS. My question is, where does cystine go and what is it used for after leaving lysosomes? And is it ever broken further into monomers of cysteine?

I also read about a term "cysteine protease". What is its relation to cystine?


I'm not sure as to what level of detail I should answer this question, but I will try to give you as much information as I can that I think will be relevant to.. whatever it is you are doing.

Cysteine proteases

Are a class of enzymes that degrade proteins, and utilise cysteine-residues as part of their catalytic mechanism.


Is, as you said, a cysteine-dimer.. Which basically means two cysteines stuck together.

These are often formed in the intermediate steps of a cysteine protease's catalytic mechanism.

But also, during protein degradation within the lysosome, cystine can be released as a free molecule. When this happens, it will be transported in to the cytoplasm, to be processed via anabolic pathways into various products, such as glutathione (an antioxidant).

So, basically, the function of your seven-transmembrane (7TM) transporter protein is:

To make accessible to the cell, the digested products of proteins (or, amino acids).

Cystine is an amino acid. Cysteine is also an amino acid. These can all be used by a cell to create new molecules, such as proteins, or antioxidants, or vitamins or cofactors, etc., so many things. And can indeed be broken down in to cysteine.

The function of the CTNS gene, is to provide the cell with the genetic information used to synthesise the 7TM-transporter protein.

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  • $\begingroup$ Thank you very much for the reply. Two followup questions: 1) When you say "To make accessible to the cell, the digested products of proteins (or, amino acids)", you mean specifically cysteine (or cystine), right? Other aminoacids are made accessible to the cell by other transporters? 2) When you say "Cysteine proteases are a class of enzymes that degrade proteins", do you mean "degrade any protein", or are specific type of proteins degraded by specific enzymes? $\endgroup$ – user5054 Aug 28 '17 at 4:47
  • $\begingroup$ 1) Based on the structure of cystine, I would guess that this particular transporter is quite specific to cystine.. But I honestly am not sure. Sorry, I usually try to do some research before answering questions on here since I do not actually know a whole lot about all of the questions that I answer.. but since it didn't appear that this would be needed of me, I did not do this before answering this time. $\endgroup$ – Bob Aug 28 '17 at 4:47
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    $\begingroup$ 2) Cystein proteases are enzymes that degrade proteins. A good example of this would be papain - which comes from papaya. Often used as a meat tenderiser. And, also found in pineapple juice - if you were to take a steak and put a few drops of pineapple juice on it, and then leave it for a while, you would come back to find the steak a gooey mess of "mush".. for lack of a better term. $\endgroup$ – Bob Aug 28 '17 at 4:51
  • $\begingroup$ Thanks! One more dumb question: If cystein proteases degrade any protein, why is there a specific "cysteine" in the name, given cysteine is a specific aminoacid? Sorry that those may look like random questions, but they are in fact not. There is a paper titled "Cysteine protease inhibitors effectively reduce in vivo levels of brain beta-amyloid related to Alzheimer's disease", and I am trying to make clear the potential relation of cysteine and amyloid beta in my mind. $\endgroup$ – user5054 Aug 28 '17 at 5:01
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    $\begingroup$ Well, as I said, "Cysteine proteases ... utilise cysteine-residues as part of their catalytic mechanism." Proteases are proteins! =D And proteins are made up of amino-acid-residues.. So basically, the cysteine residues are the amino acids within the protease that are responsible for catalysis. Another way to say this is: "Cysteine proteases utilize cysteine as their catalytic residues".....Does this make more sense? $\endgroup$ – Bob Aug 28 '17 at 5:06

The cytosol is maintained as a reducing environment and so disulfides are readily broken. The resulting cysteine can then be metabolised in a number of pathways:

enter image description here

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