Mitochondrial protein import

Abstract

Most mitochondrial proteins are synthesized as precursor proteins on cytosolic polysomes and are subsequently imported into mitochondria. Many precursors carry amino-terminal presequences which contain information for their targeting to mitochondria. In several cases, targeting and sorting information is also contained in non-amino-terminal portions of the precursor protein. Nucleoside triphosphates are required to keep precursors in an import-competent (unfolded) conformation. The precursors bind to specific receptor proteins on the mitochondrial surface and interact with a general insertion protein (GIP) in the outer membrane. The initial interaction of the precursor with the inner membrane requires the mitochondrial membrane potential (delta psi) and occurs at contact sites between outer and inner membranes. Completion of translocation into the inner membrane or matrix is independent of delta psi. The presequences are cleaved off by the processing peptidase in the mitochondrial matrix. In several cases, a second proteolytic processing event is performed in either the matrix or in the intermembrane space. Other modifications can occur such as the addition of prosthetic groups (e.g., heme or Fe/S clusters). Some precursors of proteins of the intermembrane space or the outer surface of the inner membrane are retranslocated from the matrix space across the inner membrane to their functional destination ('conservative sorting'). Finally, many proteins are assembled in multi-subunit complexes. Exceptions to this general import pathway are known. Precursors of outer membrane proteins are transported directly into the outer membrane in a receptor-dependent manner. The precursor of cytochrome c is directly translocated across the outer membrane and thereby reaches the intermembrane space. In addition to the general sequence of events which occurs during mitochondrial protein import, current research focuses on the molecules themselves that are involved in these processes.

Protein import into chloroplasts

“…After recognition by proteinaceous receptors on the outer envelope membrane surface, the proteins are translocated into and across the envelope membranes. ATP hydrolysis is required at least at two distinct steps during the import pathway while a membrane potential necessary in mitochondrial protein transloca tion is not requisite in plastids (6,7). Low concentrations (5-100 μΜ) ATP are required for binding (8), but higher concentrations (0,2-2 mM) to completely translocate the precursor protein into the plastid (9)(10)(11)(12) (18).…”

Duplication of leader sequence for protein targeting to mitochondria leads to increased import efficiency

“…This approach has been especially well developed concerning protein translocation into mitochondria, where chimaeric fusions can be used to deliver passenger proteins into the organelle both in vitro and in vivo [2,3]. For example the soluble cytosolic protein dihydrofolate reduetase can be readily targeted into mitochondria when fused to one of various leaders from naturally imported proteins (reviewed in [1][2][3]). Our laboratory has adopted such an approach for studying the structure and function of mitochondrially encoded proteins.…”

Mitochondrial protein import: Specific recognition and membrane translocation of preproteins

“…Correspondence to: W. Neupert mitochondria and sorted to the four subcompartments: the outer and the inner membranes, the intermembrane space and the matrix (Attardi & Schatz, 1988;Hartl et al, 1989).…”

“…Interestingly, this protease shares sequence similarity with the E. coli leader peptidase supporting the endosymbiontic theory. Some of the intermembrane space precursor proteins contain a bipartite signal at their amino-terminus (Gasser et al, 1982;Kaput, Goltz & Blobel, 1982;Teintze et al, 1982;Hurt & van Loon, 1986;Hartl et al, 1989). This signal initially consists of a positively charged matrix-targeting sequence, followed by a hydrophobic sorting signal which bears some resemblance to bacterial leader sequences (Randall & Hardy, 1989).…”

“…An important step towards a molecular understanding of mitochondrial protein uptake was the establishment of methods to arrest translocation at certain stages and thus generate translocation intermediates (Pfanner & Neupert, 1987;Hartl et al, 1989;Planner et al, 1990). In particular, analysis of the import of the ADP/ATP carrier provided useful information on the consecutive steps of translocation.…”