Question originally asked on Quora. These proteins have many functional similarities, so why do cells need both to handle unfolded proteins?
Hsp70 and Hsp90 are not single proteins, but entire protein families. And those two protein families are not the only ones of their kind. There are different groups of heat shock proteins, Hsp70 and Hsp90 are molecular chaperones that assist in protein folding. There are five major families of molecular chaperones: Hsp100, Hsp90, Hsp70, Hsp60 and Hsps.
Hsp70 binds to nonnative, unstructured segments of proteins and helps folding them. Hsp90 doesn't bind to unfolded proteins, but to native-like proteins. Some substrates are also handed over to Hsp90 by Hsp70.
The whole area of chaperones and heat shock response is a very, very large and interesting field. I recommend to you to read the reviews below.
Often cells have multiple types of the same protein — this redundancy can have different effects for different requirements such as having proteins function under different physiological conditions, or providing specificity to a certain class of ligand proteins or so on.
But here, it seems like the two have some synergistic interaction, a tag team if you will.
Unfortunately this article's full version can only be accessed if you're at a university or somewhere that has a subscription to some of the large research databases, but the abstract is free and it may provide more clarification.