Group II chaperonins: New TRiC(k)s and turns of a protein folding machine

In the past decade, the eubacterial group I I chaperonin GroEL became the p aradigm of a protein folding machine. More recently, electron microscopy an d X-ray crystallography offered insights into the structure of the thermoso me, the archetype of the group II chaperonins which also comprise the chape ronin from the eukaryotic cytosol TRiC. Some structural differences from Gr oEL were revealed, namely-the existence of a built-in lid provided by the h elical protrusions of the apical domains instead of a GroES-like co-chapero nin. These structural studies provide a framework for understanding the dif ferences in the mode of action between the group II and the group:I chapero nins. In vitro analyses' of the folding of non-native substrates coupled to ATP binding and hydrolysis are progressing towards establishing a function al cycle for group II chaperonins. A protein complex called GimC/prefoldin has recently been found to cooperate with TRiC in vivo, and its characteriz ation is under way. (C) 1999 Academic Press.