title: Import of rat mitochondrial citrate carrier (CIC) at increasing salt concentrations promotes presequence binding to import receptor Tom20 and inhibits membrane translocation.
abstract: Mitochondria contain a family of related carrier proteins that mediate transport of metabolites across the mitochondrial inner membrane. All members of this family are synthesized in the cytosol. We characterized the interactions of newly synthesized rat citrate carrier (CIC) precursor protein (pCIC) with the components of the mitochondrial protein import machinery. pCIC contains both a positively charged presequence of 13 amino acids and internal targeting sequences. We found that the pCIC presequence does not interfere with the import pathway and merely acts as an internal chaperone in the cytosol. Under conditions of increased ionic strength, the pCIC presequence binds to the import receptor Tom20 and accumulates at the mitochondrial surface, thereby delaying pCIC translocation across the mitochondrial outer membrane. Similarly, the presequence of the bovine phosphate carrier (PiC) precursor protein (pPiC) is arrested at the mitochondrial surface when salt concentrations are elevated. We conclude that presequences can only act as mediators of mitochondrial protein import if they allow rapid release from import receptor sites. Release from receptors sites may be rate-limiting in translocation.
authors.affiliation: Dipartimento di Scienze e Tecnologie Biologiche ed Ambientali, Università di Lecce, I-73100 Lecce, Italy. vincenzo.zara@unile.it
authors.forename: Vincenzo
authors.initials: V
authors.lastname: Zara
authors.forename: Alessandra
authors.initials: A
authors.lastname: Ferramosca
authors.forename: Panagiotis
authors.initials: P
authors.lastname: Papatheodorou
authors.forename: Ferdinando
authors.initials: F
authors.lastname: Palmieri
authors.forename: Joachim
authors.initials: J
authors.lastname: Rassow
mesh_terms: D000818:Animals; D002352:Carrier Proteins / Q000235:genetics / Q000378:metabolism*; D002417:Cattle; D018840:HSP70 Heat-Shock Proteins / Q000378:metabolism; D006801:Humans; D026901:Membrane Transport Proteins; D008930:Mitochondria, Liver / Q000378:metabolism*; D033681:Mitochondrial Membrane Transport Proteins; D000090622:Mitochondrial Precursor Protein Import Complex Proteins; D024101:Mitochondrial Proteins / Q000235:genetics / Q000378:metabolism*; D009994:Osmolar Concentration; D011485:Protein Binding; D011498:Protein Precursors / Q000235:genetics / Q000378:metabolism*; D021381:Protein Transport / Q000502:physiology*; D051381:Rats; D011956:Receptors, Cell Surface; D018160:Receptors, Cytoplasmic and Nuclear / Q000235:genetics / Q000378:metabolism*; D029701:Saccharomyces cerevisiae Proteins / Q000378:metabolism; D012492:Salts / Q000737:chemistry*
chemical_list: D002352:Carrier Proteins; D018840:HSP70 Heat-Shock Proteins; D026901:Membrane Transport Proteins; D033681:Mitochondrial Membrane Transport Proteins; D000090622:Mitochondrial Precursor Protein Import Complex Proteins; D024101:Mitochondrial Proteins; D011498:Protein Precursors; D011956:Receptors, Cell Surface; D018160:Receptors, Cytoplasmic and Nuclear; D029701:Saccharomyces cerevisiae Proteins; D012492:Salts; C489350:TOM20 protein, S cerevisiae; C489998:Tomm20 protein, rat; C467795:Tomm70 protein, rat; C020723:citrate-binding transport protein