Cleaving transmembrane proteins

Hey year 13 reading about Alzheimer's and one of the hypotheses relating to cause is that of mutations of genes encoding for PS-1 and PS-2, two proteins which function as part of the "gamma-secretase transmembrane protease complex". That's basically a complex whose function is cleaving ("cutting up" or breaking down I guess) transmembrane proteins, referred to as intrinsic proteins at A level.

My question is why is there the need to break down these proteins? I'm not sure if they're the proteins of the same cell containing the gamma-secretase complex or if they maybe just play a role in digestion.

Could anyone help? >.<

All proteins in your body have to be broken down at one time or another! Proteins, due to their structural function, undergo a lot of stress and damage, and also have their own age-related breakdown in function. So your body is in constant turnover of breaking down your proteins and rebuilding them again.

However, this isn't the case with the proteins you're talking about. After you put a protein together and let it fold up into the right shape, it's not always ready to go from the get go! Some proteins need to undergo some processing even after they've folded up (like adding bits on or chopping bits off): this is the case with Amyloid precursor protein. Normally, what would happen, is that the (alpha and beta) secretase complex would chop up the APP into a functional forum. But in Alzheimer's, a proposed mechanism is that the (gamma) secretase complex chops up the APP in a wrong way, which leads to the formation of a misfolded, dysfunctional amyloid protein. This collection of amyloid (called amyloid plaques) is proposed as one of the causes of Alzheimer's, but we're a bit confused about it! Not all brains with amyloid plaque build-ups have Alzehimer's, and not all brains with Alzheimer's have amyloid plaque build-ups! We don't fully understand how Alzehimer's come about, and as you'd expect, we don't fully understand these amyloid proteins and the secretase complexes either!