The key factors for protoss are:
Protoss nuke defense is probably the hardest to do. Observers can be blinded, cannons destroyed,and you may have no high templars. This is why its very important that you don't allow them to get nukes, constantly harass their minrals or expansion(s). However if you cannot stop them from getting them then try and follow this.
Observers- The only thing observers have going for them is that they're cloaked but if someone brings in a science vessel with medics they're useless (blind). Therefore you may have to mass produce em i'd say about 5-6 in your base will do, for some this may be overkill but at least u know the nuke will never land. This is one way however if you have high templars and only like 2 observers you can use psi storm to kill the ghost. Pay attention to your observers that u use to go find the ghost and even if it reveals him for a moment it'll b worth it cuz then you can send in your high templar and psi storm him.
Cannons- Cannons serve a higher purpose than terran missile turrets since they can attack the ground. However they are by all means not the greatest deterrent from bieng nuked cuz all the terran player has to do is simply get ocular implants or just avoid the damn thing. Therefore you do basically the same thing as terran nuke defense and place them randomly around your base.