What are the most important differences between HSP70 and HSP90?
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.




Vabulas, R. M., Raychaudhuri, S., Hayer-Hartl, M. &amp; Hartl, F. U. Protein folding in the cytoplasm and the heat shock response. Cold Spring Harb Perspect Biol 2, (2010).
Richter, K., Haslbeck, M. &amp; Buchner, J. The heat shock response: life on the verge of death. Mol. Cell 40, 253-266 (2010).