Ungermann C, Guiard B, Neupert W, Cyr D M
Institut für Physiologische Chemie der Universität München, Germany.
EMBO J. 1996 Feb 15;15(4):735-44.
New steps in the reaction cycle that drives protein translocation into the mitochondrial matrix have been defined. The membrane potential (delta psi)- and the mtHsp70/MIM44-dependent import machinery cooperate in the transfer of the presequence across the inner membrane. Translocation intermediates, arrested at a stage where only the presequence could form a complex with mtHsp70, still required delta psi for further import. Delta psi at this stage prevented retrograde movement, since mtHsp70 did not bind to the presequence with sufficient affinity. In contrast, mature regions of incoming chains adjacent to the presequence were bound by mtHsp70 tightly enough to stabilize them in the matrix. Cycling of the mtHsp70 on and off incoming chains is a continuous process in the presence of matrix ATP. Both MIM44-bound and free forms of mtHsp70 were found in association with the incoming chains. These data are consistent with a reaction pathway in which the mtHsp70/MIM44 complex acts as a molecular ratchet on the cis side of the inner membrane to drive protein translocation into the matrix.
驱动蛋白质转运至线粒体基质的反应循环中的新步骤已被确定。膜电位（Δψ）以及依赖线粒体热休克蛋白70（mtHsp70）/MIM44的转运机制共同作用，将前导序列转运穿过内膜。转运中间体在仅前导序列能与mtHsp70形成复合物的阶段停滞，进一步转运仍需要Δψ。此时的Δψ可防止逆向运动，因为mtHsp70与前导序列的结合亲和力不足。相反，靠近前导序列的输入链成熟区域被mtHsp70紧密结合，足以使其在基质中稳定。在基质ATP存在的情况下，mtHsp70在输入链上的循环结合和解离是一个持续的过程。发现与输入链相关的既有与MIM44结合的mtHsp70形式，也有游离形式。这些数据与一种反应途径相符，即mtHsp70/MIM44复合物在内膜顺式侧作为分子棘轮，驱动蛋白质转运至基质。
