I apologize for the analogy if you are really interested in cars. This is about protein function, but a crucial one because the extraction from membranes of a mistargeted protein is an essential component of the quality control mechanism. In a recent publication, the P5-ATPases ( in this case the yeast Spf1p, the red Ferrari) was proposed to dislocate mistargeted membrane proteins from ER. Spf1p is thought to be a beautiful single subunit ATPase running at about 0.1 min-1. However, Spf1p is not the fastest model made by the Cavaliere Enzo, since the related sodium-potassium pumping ATPase runs at about 2000 min-1. Of course, dislocating a membrane domain may take some more time than translocating ions. The point is that this is the first P-ATPase proposed to have a peptide as a substrate. I believe that the extraction of peptides from membranes is usually performed by more complex and slow machines AAA+ ATPases like the mitochondrial Msp1 of yeast (je je the Oldsmobile). So both Spf1p and Mspf1 have the same function although at different membranes? I have no doubts about the Ferrari, but why would you choose an Oldsmobile if the cost is not a limiting factor?
I would assume that the lipid environment from which the peptides are extracted represent an important contribution to the energetic cost of the extraction process. Another important contribution of course would be from the peptides themselves, but the physicochemical properties of both play into the energetic cost of the process - ie. it would take more energy to seperate a highly hydrophobic peptide from a highly hydrophobic environment (maybe with no charged lipid headgroups, high in cholesterol, low protein density, etc.) as compared to a less hydrophobic peptide from a less hydrophopbic environment rich in charged lipids etc. Maybe more light could be shed on this in the future, as the said publication did not reveal the sequence of what was trapped inside Spf1p and no biochemical assay was used to identidy an optimal peptide ligand.
So to answer your question with the analogy of cars. Maybe the difference between Spf1p and AAA+ ATPases is more like a ferrari versus a jeep? Each is specialized for the same function (driving), but is optimized to do so under different circumctances (road versus terrain). If this is true that would imply that "cost" indeed is a factor, but it is not the economic cost rather the energetic cost.
I think this would explain why Spf1p have been observed in highly localized regions of the ER membrane.
Hi Danny, thank you for your answer. As always you provide an interesting point of view y like the jeep analogy, I should change the title. Hit the road Spf1p!
Well, you posed an interesting question as well. Maybe that would be an interesting topic for further research... a closer examination of the peptide sequence and lipid environment on actual ATPase activity as well as peptide release? with the tools at hand that would certainly be a feasible direction :)
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