How is GPCRs expressed.

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Jama
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How is GPCRs expressed.

During the expression of GPCRs and before the incorporation of these large 7 Tranmembrane receptors into the lipid structures do they exits freely in the cytoplasm? I want to know because I think a molecular I’m studying is binding to one of the extracellular loops – is this biologically possible?
 
Thanks allot guys

Jama

Sami Tuomivaara
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Jama,

Jama,

To answer your first question, GPCRs are never found freely in aqueous surroundings. They are inserted in the endoplasmic reticulum co-translationally during their synthesis. This occurs due to the intrinsic membrane propensity of the polypeptide itself and some helper proteins in the ER lumen. Thus, GPCRs are synthesised and folded in and through the membrane simultaneously. Vesicles bud off from the ER (with GPCR on the vesicle membrane), migrate to Golgi apparatus, where it merges to the Golgi apparatus membrane. Then, from Golgi, vesicle buds off again (with GPCR on it) to migrate to the extracellular membrane and fuse with it. All this time, GPCRs are firmly within a lipid bilayer.

Loop regions though are in contact with the cytosol, and the interior of the vesicle/organelle, and the extracellular space. If you look at the topology of GPCRs, loops are found in both sides of the cell membrane. So to answer your second question, interaction of GPCR extracellular loop with small and large molecules in the extracellular space is very much biologically possible. It has to be said that the paradigm is that the binding site that confers the selectivity for small molecules is usually within a cavity formed by the membrane-spanning helices. In the case of large proteinous ligands, extracellular loops and the N-terminal "hanging" terminus are the binding sites, whereas for smaller peptides both loops and the cavity participate in the binding.

Cheers,

The FFM
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Inside the cell, the

Inside the cell, the extracellular loops of a GPCR are located in the lumen of the ER, Golgi or the vesicles that traffic them to the plasma membrane.

So if the molecule you are studying is a small membrane permeable ligand then as Suola said it may well be possible for it to bind to a recognition motif that is partially or completely formed by an extracellular region of the GPCR.

However, if the molecule that binds your GPCR extracellular loop is a cytoplasmic protein I would be  suspicious unless you know it is something that can be found in the lumen of ER or vesicles.

Sami Tuomivaara
Sami Tuomivaara's picture
TheFFM,

TheFFM,

Thanks for clarification, I didn't make the link between the two questions :-( and missed the whole point... Of course cytoplasmic, but not cytosolic protein can bind to the extracellular loop(s) of GPCR.

Cheers,