Yeast cytosol has two important co-chaperons; Ydj1 and Sis1. Genetic experiments showed that Ydj1 is not I essential for viability; however, cells lacking it grow very poorly at 30 degrees C or unable to grow at extreme temperatures. On the other hand, Sis1 is an essential protein and apparently plays a functional role at assembly or disassembly of protein complexes. Stability experiments revealed that only Ydj1-protected Hsp70 proteins can hydrolyze ATP under prolonged stress. (c) 2007 Wiley Periodicals, Inc.