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Transport Proteins in Plasma Membrane

  1. Sep 30, 2003 #1
    According to my textbook (again, this might not be really accurate because of translation):
    Now, here is my question :
    If proteins are just molecules (big ones i guess), how do they 'know' what the cell needs at a certain time (to let it into the cell) ?
    Thanks in advance.
  2. jcsd
  3. Sep 30, 2003 #2


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    It does not know if certain molecules but the cell knows what it requires. Some receptor/transport protein are express all the time whereas some are express under certain condition.

    For example, iron is require all the time for basic cell function, therefore free iron will be transport in on a regular basis. For pathogen, blood is poor in free Fe but some protein are bound to some other compound such as haem, haemoglobin, transferin, etc. In this iron restricted condition, pathogenic bacteria will express certain receptor/transport protein that can bind/transport Fe bound by haem, haemoglobin, and/or transferin.

    Edit: So the cell knows that it requires certain molecules by monitoring the level in the cytoplasm. If iron drops than iron transport protein will be express at a higher level through Fe related regulatory protein. If iron increasse to the desired level than the regulatory protein shut down the production of the transport protein.
    Last edited: Sep 30, 2003
  4. Sep 30, 2003 #3
    So let me see if i understood (personally, i don't think i did).
    For each compuond/element that the cell needs (let's say, a compund called X), there is a regulatory protein in the cytoplasm that check if there is enough of the X molecules in the cytoplasm.
    If there aren't enough of X molecules in the cytoplasm, this protein builds the X tranport protein ?

    If my understanding is right, then some new questions will raise :
    1-How can a protein build another protein ? doesn't it need somekind of genetic information to know how to build it ? (mRNA i guess ?)
    2-As far as i remember, the transport protein are not used while transporting, in other words, after the transport protein have transported the molecule of X, it will still be ready to transport another molecule of X, so how can we stop the tranport protein from transporting new X molecules when the cell does not need any new X molecules anymore ?
    3-If the transport molecules are built inside the cytoplasm, how do they head to the membrane ?

    I hope you will be patient with me, i am only a school student.
  5. Sep 30, 2003 #4
    There are lots of different types of regulation and signalling and transporters. Using many different methods. Understand that proteins have all sorts of substructures, and different binding sites. They can even have different subunits. Often proteins can be complexed together. A very interesting category is call G proteins, which I highly encourage looking up.

    This may not be how it works in every case, but these kind of things happen: Let's say the cell is deficient in X, and starts getting sick, or whatever it is that happens to the cell when it doesn't get enough X. Say, for example, because there is not enough X, there is a build up of Y. Regulatory protein A says "oh crap, there's too much Y" so Regulatory protein A releases Subunit B. Subunit B floats around the membrane until it runs into Transport protein M (which is turned off). Subunit B binds to transporter M and turns it on, which does its thing, X is pumped into the cell, the concentration of Y goes down, Reg. Protein A stops releases Subunit B, and Transporter M eventually shuts down, and the concentration of X is restored.

    To answer your questions:

    1. no proteins don't build new proteins themselves, but protein complexes can release signalling proteins and proteins can promote transcription and translation, I believe.

    2. Some transporters can be turned on and off, some are just passive and allow the flow along a concentration gradient. The above example illustrate how it can be turned on and off.

    3. This is a very good question. How proteins are packaged and delivered is a huge topic and subject of a great deal of research. I can tell you that a lot of transport proteins are cylindrical, the middle is chemically attraced to the interior of the membrane while the ends are attracted to the inside and outside of the cell. So sitting right inside the membrane is the most natural place for them to be.
  6. Sep 30, 2003 #5


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    There is a signal peptide in the amino acid that tell twhere protein goes. In eukaryotes there is signal peptide for mitochondria, membrane and other organelles. In bacteria and archea, the most signals are for transport to the membrane.
    Therefore the theoritical length of the amino acid sequence is different than the amino acid sequence of the isolaled protein. The signal is cleave after the protein get to its destination.

    for the rest I think chemicalsuperfreak gave good explanation.

    It is why we are here.
  7. Sep 30, 2003 #6


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    Let me elaborate a little further on the mechanism. There are two types of transportation mechanisms: active and passive.

    No energy is required for passive transport and will be performed automatically, where the molecule is transported along its gradient. The receptor is basically a hollow tube through which the molecule fits.. this process is not very selective.

    It is also possible to imagine a receptor in the shape of a V, the opening is on the extracellular side, where there is a domain which very specifically can bind molecules that are needed in the cell. If that molecule is needed INSIDE the cell, the cell will signal the receptor with an intracellular messenger, which will bind to the point of the V. This causes a conformational change and essentially pulls the molecule into the cell, if you understand.

    With this type of receptor it is possible to transport a molecule against its gradient (but will require input of energy).

    Chemical superfreak, aren't G-proteins used for signaling and not necessarily the uptake of molecules as nutrients?
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