Mechanisms

From Chaperome

Please submit your thoughts and insite here (click edit above)

Models of Hsp70 action in Protein Translocation

The mechanism of Hsp70 action in the movement of preproteins through the translcoation channel is a subject of great debate. If mitochondrial Hsp70 activity is blocked by depleting matrix ATP, preproteins are able to slide back and forth in the import channel (Schwarz et al., 1993). The mechanism by which Hsp70 provides unidirectional movement of the preprotein into the matrix is unclear. Two models, which are not mutually exclusive, have been proposed to describe the molecular mechanism of Hsp70 action in the import of preproteins into the matrix of mitochondria: the Brownian motion (trapping) model and the Motor (pulling) model (Neupert, 1997).

The Brownian motion model asserts that translocation of preproteins into the matrix is a spontaneous event (Gaume et al., 1998; Schneider et al., 1994; Ungermann et al., 1994). Domains on the cytosolic face of the mitochondrial outer membrane spontaneously unfold or "breathe" allowing segments of the protein to enter the import channel. The preprotein in transit is free to move back and forth in the channel based on Brownian motion. Once the preprotein emerges from the import channel into the matrix, Hsp70 binds the preprotein and prevents its backslipping. The DnaJ domain containing protein Tim44 is thought to recruit Hsp70 to the import where Hsp70 can interact with the emerging preprotein. Further sponataneous unfolding of folded domains on the cytosolic face of the mitochondrial outer membrane allows additional segments of the preprotein to enter the translocation channel, exposing a new binding stite for another molecule of Hsp70. Thus, the binding of Hsp70 traps the preprotein and prevents its backslipping thereby providing a molecular mechanism for unidirectional movement of the preprotein into the matrix. According to the Brownian Motion model, the translocation of preproteins is efficient if Hsp70 is available at the import channel to bind the preprotein as it emerges into the matrix. According to this model, the role of the DnaJ domain containing protein Tim44 is to recruit Hsp70 to the import channel to provide a high local concentration of Hsp70 for efficient import.

The Motor (pulling) model describes a more active role for Hsp70 in the process of translocation. After the preprotein emerges from the import channel, Tim44-bound Hsp70 binds the preprotein and undergoes a conformational change upon hydrolysis of ATP. Using Tim44 as a membrane anchor, the conformational change of Hsp70 generates a force that will labilize folded domains of the preprotein on the cytosolic side of the outer mitochondrial membrane (Horst et al., 1996, Kronidou et al. 1994, Matouschek et al. , 1997). Thus, Hsp70's pulling force on the protein in transit drives the preprotein into the matrix (Glick, 1995). The interaction of Tim44 and Ssc1 is required for the power stroke of the motor model.

see also Core chaperone families