Biogenesis of Tim proteins of the mitochondrial carrier import pathway: differential targeting mechanisms and crossing over with the main import pathway.
Two major routes of preprotein targeting into mitochondria are known. Preproteins carrying amino-terminal signals mainly use Tom20, the general import pore (GIP) complex and the Tim23-Tim17 complex. Preproteins with internal signals such as inner membrane carriers use Tom70, the GIP complex, and the special Tim pathway, involving small Tims of the intermembrane space and Tim22-Tim54 of the inner membrane. Little is known about the biogenesis and assembly of the Tim proteins of this carrier pathway. We report that import of the preprotein of Tim22 requires Tom20, although it uses the carrier Tim route. In contrast, the preprotein of Tim54 mainly uses Tom70, yet it follows the Tim23-Tim17 pathway. The positively charged amino-terminal region of Tim54 is required for membrane translocation but not for targeting to Tom70. In addition, we identify two novel homologues of the small Tim proteins and show that targeting of the small Tims follows a third new route where surface receptors are dispensable, yet Tom5 of the GIP complex is crucial. We conclude that the biogenesis of Tim proteins of the carrier pathway cannot be described by either one of the two major import routes, but involves new types of import pathways composed of various features of the hitherto known routes, including crossing over at the level of the GIP.
Institut für Biochemie und Molekularbiologie, Universität Freiburg, D-79104 Freiburg, Germany.
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M
Kurz
H
H
Martin
J
J
Rassow
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N
Pfanner
M T
MT
Ryan
D000595:Amino Acid Sequence; D001692:Biological Transport; D002352:Carrier Proteins / Q000235:genetics / Q000378:metabolism*; D005656:Fungal Proteins / Q000235:genetics / Q000378:metabolism; D008565:Membrane Proteins / Q000235:genetics / Q000378:metabolism*; D026901:Membrane Transport Proteins; D008928:Mitochondria / Q000187:drug effects / Q000378:metabolism*; D033681:Mitochondrial Membrane Transport Proteins; D000090622:Mitochondrial Precursor Protein Import Complex Proteins; D008969:Molecular Sequence Data; D017354:Point Mutation; D011956:Receptors, Cell Surface / Q000378:metabolism; D018160:Receptors, Cytoplasmic and Nuclear; D012097:Repressor Proteins; D029701:Saccharomyces cerevisiae Proteins; D014357:Trypsin / Q000378:metabolism / Q000494:pharmacology; D015003:Yeasts / Q000235:genetics / Q000378:metabolism
D002352:Carrier Proteins; D005656:Fungal Proteins; D008565:Membrane Proteins; D026901:Membrane Transport Proteins; D033681:Mitochondrial Membrane Transport Proteins; D000090622:Mitochondrial Precursor Protein Import Complex Proteins; D011956:Receptors, Cell Surface; D018160:Receptors, Cytoplasmic and Nuclear; D012097:Repressor Proteins; D029701:Saccharomyces cerevisiae Proteins; C091191:TIM17 protein, S cerevisiae; C102549:TIM22 protein, S cerevisiae; C083081:TIM23 protein, S cerevisiae; C110036:TIM54 protein, S cerevisiae; C489350:TOM20 protein, S cerevisiae; C090857:TOM22 protein, S cerevisiae; C106794:TOM5 protein, S cerevisiae; C489039:TOM70 protein, S cerevisiae; D014357:Trypsin