Tag Archives: Beta sheet

Does gamma-secretase have sex with its substrates?

This is a family blog (for the most part), so discretion is advised in reading further.   Billions have been spent trying to inhibit gamma-secretase.  Over 150 different mutations have been associated with familial Alzheimer’s disease.  The more we know about the way it works, the better.

A recent very impressive paper from China did just that [ Science vol. 363 pp. 690- 691, 701 eaaw0930 pp. 1 –> 8 ’19 ].

Gamma secretase is actually a combination of 4 proteins (presenilin1, nicastrin, APH1 (anterior pharynx defect) and PEN-2 (presenilin enhancer 2). It is embedded in membranes and has at least 19 transmembrane segments.  It cleaves a variety of proteins spanning membranes (e.g it hydrolyzes a peptide bond — which is just an amide).  The big deal is that cleavage occurs in the hydrophobic interior of the membrane rather than in the cytoplasm where there is plenty of water around.

Gamma secretase cleaves at least 20 different proteins this way, not just the amyloid precursor protein, one of whose cleavage products is the Abeta peptide making up a large component of the senile plaque of Alzheimer’s disease.

To get near gamma secretase, another enzyme must first cleave APP in another place so one extramembrane fragment is short.  Why so the rest of the protein can fit under a loop between two transmembrane helices of nicastrin.  This is elegance itself, so the gamma secretase doesn’t go around chopping up the myriad of extracellular proteins we have.

The 19 or so transmembrane helices of the 4 gamma secretase proteins form a horseshoe, into which migrates the transmembrane segment of the protein to be cleaved (once it can fit under the nicastrin loop).

So why is discretion advised before reading further?  Because the actual mechanism of cleavage involves intimate coupling of the proteins.    One of the transmembrane helices of presenilin1 unfolds to form two beta strands, and the transmembrane helix of the target protein unfolds to form one beta strand, the two strands pair up forming a beta sheet, and then the aspartic acid at the active site of gamma secretase cleaves the target (deflowers it if you will).  Is this sexual or what?

All in all another tribute to ingenuity (and possibly the prurience) of the blind watchmaker. What an elegant mechanism.

Have a look at the pictures in the Science article, but I think it is under a paywall.

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