The protein import motor of mitochondria: a targeted molecular ratchet driving unfolding and translocation.
Unfolding and import of preproteins into mitochondria are facilitated by a molecular motor in which heat shock protein 70 (Hsp70) in the matrix plays an essential role. Here we present two different experimental approaches to analyze mechanisms underlying this function of Hsp70. First, preproteins containing stretches of glutamic acid (polyE) or glycine (polyG) repeats in front of folded domains were imported into mitochondria. This occurred although Hsp70 cannot pull on these stretches to unfold the folded domains, since it does not bind to polyE and polyG. Secondly, preproteins containing titin immunoglobulin (Ig)-like domains were imported into mitochondria, despite the fact that forces of >200 pN are required to mechanically unfold these domains. Since molecular motors generate forces of approximately 5 pN, Hsp70 could not promote unfolding of the Ig-like domains by mechanical pulling. Our observations suggest that Hsp70 acts as an element of a Brownian ratchet, which mediates unfolding and translocation of preproteins across the mitochondrial membranes.
Institut für Physiologische Chemie der Universität München, Butenandtstrasse 5, D-81377 München, Germany.
Koji
K
Okamoto
Achim
A
Brinker
Stefan A
SA
Paschen
Ismail
I
Moarefi
Manajit
M
Hayer-Hartl
Walter
W
Neupert
Michael
M
Brunner
D000595:Amino Acid Sequence; D018840:HSP70 Heat-Shock Proteins / Q000378:metabolism; D008928:Mitochondria / Q000378:metabolism*; D011485:Protein Binding; D011489:Protein Denaturation; D021381:Protein Transport; D020349:Surface Plasmon Resonance
D018840:HSP70 Heat-Shock Proteins