This report provides a very thorough and systematic analysis of the role of the various transmembrane domains of PS1 in γ-secretase assembly, stability and activation. The evidence largely supports the role of specific TM domains in the various steps in the maturation of the protease complex as laid out schematically in Figure 9. This includes the role of TM2 and the luminal region of TM6 in the formation of the initial substrate binding site: swapping these regions with those of another unrelated membrane protein leads to the inability of a photoaffinity helical peptide probe targeted to this initial substrate binding site from labeling PS1. These two TM regions are apparently critical to the formation of this binding site, but whether they are both direct contributors to the binding site is not clear. It could be, for instance, that TM2 is not part of the initial substrate binding site but can nevertheless affect the conformation of this binding site. Some chemical crosslinking evidence is provided that TM2 is near TM9, the latter previously being implicated in binding and...  Read more

This report provides a very thorough and systematic analysis of the role of the various transmembrane domains of PS1 in γ-secretase assembly, stability and activation. The evidence largely supports the role of specific TM domains in the various steps in the maturation of the protease complex as laid out schematically in Figure 9. This includes the role of TM2 and the luminal region of TM6 in the formation of the initial substrate binding site: swapping these regions with those of another unrelated membrane protein leads to the inability of a photoaffinity helical peptide probe targeted to this initial substrate binding site from labeling PS1. These two TM regions are apparently critical to the formation of this binding site, but whether they are both direct contributors to the binding site is not clear. It could be, for instance, that TM2 is not part of the initial substrate binding site but can nevertheless affect the conformation of this binding site. Some chemical crosslinking evidence is provided that TM2 is near TM9, the latter previously being implicated in binding and lateral gating of substrate. However, the evidence for TM2 proximity to TM9 is rather weak, and such proximity would not mean that TM2 is necessarily part of the substrate binding site.

View all comments by Michael Wolfe

These authors take one of the helices of presenilin at a time, replace it with a helix of an unrelated protein and then measure its function. If the helix they exchange is important for a certain function, then the modified presenilin should no longer be able to show this function. It is like exchanging the individual parts of a car with, say, parts of household appliances. If you exchange the motor of the car with a vacuum cleaner and then try to start the car and it won't, then you have found out that the motor is necessary to drive the car. However if you push this car it will still move, so the motor is obviously important for driving but not in general for the ability of the car to be moved.

In essence, Watanabe et al do the same thing with presenilin and its helices. The n-terminal domain has 6 helices (the C-terminal domain, of which we have determined the structure has 3). By exchanging every domain individually, the authors found that helix2 and helix6 are involved in binding of helical peptides such as βamyloid precursor protein. They also found that helix9,...  Read more

These authors take one of the helices of presenilin at a time, replace it with a helix of an unrelated protein and then measure its function. If the helix they exchange is important for a certain function, then the modified presenilin should no longer be able to show this function. It is like exchanging the individual parts of a car with, say, parts of household appliances. If you exchange the motor of the car with a vacuum cleaner and then try to start the car and it won't, then you have found out that the motor is necessary to drive the car. However if you push this car it will still move, so the motor is obviously important for driving but not in general for the ability of the car to be moved.

In essence, Watanabe et al do the same thing with presenilin and its helices. The n-terminal domain has 6 helices (the C-terminal domain, of which we have determined the structure has 3). By exchanging every domain individually, the authors found that helix2 and helix6 are involved in binding of helical peptides such as βamyloid precursor protein. They also found that helix9, which is part of our structure, is involved in substrate binding. Understanding where the substrate binds is very important. As a field, we need lots of information about which sites bind specific substrates and which are involved in processing of APP. Only with such very detailed information will we able to develop drugs that inhibit formation of the Aβ peptide but not other functions presenilin is involved in.

In this sense both this and our paper provide complementary information and provide another small but important piece of the puzzle.

View all comments by Volker Doetsch

The main finding of this study is that using NMR studies in SDS micelles, the authors determined the structure of PS1 CTF. The structure fits well with previous studies using cysteine accessibility methods by us and by Bart de Strooper’s group. This is important because it is a first structure of a part of PS1 resolved at the atomic level. The study’s potential impact could come from an intriguing feature it shows around catalytic aspartate residue 385, that is, a half helix and extended structure kinked by a glycine residue located near the aspartate. This supports the notion that intramembrane cleavage is occurring within the water-accessible pore structure in the membrane.

At the same time, further study (e.g., using holoprotein) is required, as the PS1 N-terminal fragment (NTF) is also required for the proteolytic activity. Moreover, we have identified that TMD1 of PS1 also faces the catalytic pore (Takagi et al., unpublished result), suggesting that several TMDs in NTFs are involved in the formation of the catalytic pore.

Without an atomic structure, it is impossible...  Read more

The main finding of this study is that using NMR studies in SDS micelles, the authors determined the structure of PS1 CTF. The structure fits well with previous studies using cysteine accessibility methods by us and by Bart de Strooper’s group. This is important because it is a first structure of a part of PS1 resolved at the atomic level. The study’s potential impact could come from an intriguing feature it shows around catalytic aspartate residue 385, that is, a half helix and extended structure kinked by a glycine residue located near the aspartate. This supports the notion that intramembrane cleavage is occurring within the water-accessible pore structure in the membrane.

At the same time, further study (e.g., using holoprotein) is required, as the PS1 N-terminal fragment (NTF) is also required for the proteolytic activity. Moreover, we have identified that TMD1 of PS1 also faces the catalytic pore (Takagi et al., unpublished result), suggesting that several TMDs in NTFs are involved in the formation of the catalytic pore.

Without an atomic structure, it is impossible to “rationally design” inhibitors or modulators. The Doetsch paper is a first step toward understanding the process by γ-secretase at an atomic level. The field has encountered difficulties for structural studies of this unusual membrane-bound protease; however, we also know that tackling this issue by whatever means provides hope for the future.

View all comments by Taisuke Tomita