title: Tom20-mediated mitochondrial protein import in muscle cells during differentiation.
abstract: Mitochondrial biogenesis is accompanied by an increased expression of components of the protein import machinery, as well as increased import of proteins destined for the matrix. We evaluated the role of the outer membrane receptor Tom20 by varying its expression and measuring changes in the import of malate dehydrogenase (MDH) in differentiating C2C12 muscle cells. Cells transfected with Tom20 had levels that were twofold higher than in control cells. Labeling of cells followed by immunoprecipitation of MDH revealed equivalent increases in MDH import. This parallelism between import rate and Tom20 levels was also evident as a result of thyroid hormone treatment. Using antisense oligodeoxynucleotides, we inhibited Tom20 expression by 40%, resulting in 40-60% reductions in MDH import. In vitro assays also revealed that import into the matrix was more sensitive to Tom20 inhibition than import into the outer membrane. These data indicate a close relationship between induced changes in Tom20 and the import of a matrix protein, suggesting that Tom20 is involved in determining the kinetics of import. However, this relationship was dissociated during normal differentiation, since the expression of Tom20 remained relatively constant, whereas imported MDH increased 12-fold. Thus Tom20 is important in determining import during organelle biogenesis, but other mechanisms (e.g., intramitochondrial protein degradation or nuclear transcription) likely also play a role in establishing the final mitochondrial phenotype during normal muscle differentiation.
authors.affiliation: Department of Biology, York University, Toronto, Ontario, Canada M3J 1P3.
authors.forename: J Y
authors.initials: JY
authors.lastname: Grey
authors.forename: M K
authors.initials: MK
authors.lastname: Connor
authors.forename: J W
authors.initials: JW
authors.lastname: Gordon
authors.forename: M
authors.initials: M
authors.lastname: Yano
authors.forename: M
authors.initials: M
authors.lastname: Mori
authors.forename: D A
authors.initials: DA
authors.lastname: Hood
mesh_terms: D002454:Cell Differentiation / Q000502:physiology; D002460:Cell Line; D002462:Cell Membrane / Q000378:metabolism; D005109:Extracellular Matrix / Q000378:metabolism; D006801:Humans; D008291:Malate Dehydrogenase / Q000378:metabolism; D008565:Membrane Proteins / Q000037:antagonists & inhibitors / Q000378:metabolism / Q000502:physiology*; D026901:Membrane Transport Proteins; D008931:Mitochondria, Muscle / Q000378:metabolism*; D000090622:Mitochondrial Precursor Protein Import Complex Proteins; D009124:Muscle Proteins / Q000378:metabolism*; D009132:Muscles / Q000166:cytology* / Q000378:metabolism*; D011956:Receptors, Cell Surface; D014284:Triiodothyronine / Q000494:pharmacology
chemical_list: D008565:Membrane Proteins; D026901:Membrane Transport Proteins; D000090622:Mitochondrial Precursor Protein Import Complex Proteins; D009124:Muscle Proteins; D011956:Receptors, Cell Surface; C084008:TOMM20 protein, human; D014284:Triiodothyronine; D008291:Malate Dehydrogenase