Protein sorting between the outer and inner mitochondrial membranes: submitochondrial localization of cytochrome c1 whose presequence is replaced by the amino-terminal region of a 70 kDa outer membrane protein

Possibility of cytoplasmic pre-tRNA splicing: the yeast tRNA splicing endonuclease mainly localizes on the mitochondria

Pre-tRNA splicing has been believed to occur in the nucleus. In yeast, the tRNA splicing endonuclease that cleaves the exon-intron junctions of pre-tRNAs consists of Sen54p, Sen2p, Sen34p, and Sen15p and was thought to be an integral membrane protein of the inner nuclear envelope. Here we show that the majority of Sen2p, Sen54p, and the endonuclease activity are not localized in the nucleus, but on the mitochondrial surface. The endonuclease is peripherally associated with the cytosolic surface of the outer mitochondrial membrane. A Sen54p derivative artificially fixed on the mitochondria as an integral membrane protein can functionally replace the authentic Sen54p, whereas mutant proteins defective in mitochondrial localization are not fully active. sen2 mutant cells accumulate unspliced pre-tRNAs in the cytosol under the restrictive conditions, and this export of the pre-tRNAs partly depends on Los1p, yeast exportin-t. It is difficult to explain these results from the view of tRNA splicing in the nucleus. We rather propose a new possibility that tRNA splicing occurs on the mitochondrial surface in yeast.

Role of Arg-166 in yeast cytochrome C1

A systematic screen for dominant-negative mutations of the CYT1 gene, which encodes cytochrome c(1), revealed seven mutants after testing approximately 10(4) Saccharomyces cerevisiae strains transformed with a library of mutagenized multicopy plasmids. DNA sequence analysis revealed multiple nucleotide substitutions with six of the seven altered Cyt1p having a common R166G replacement, either by itself or accompanied with other amino acid replacements. A single R166G replacement produced by site-directed mutagenesis demonstrated that this change produced a nearly nonfunctional cytochrome c(1), with diminished growth on glycerol medium and diminished respiration but with the normal or near normal level of cytochrome c(1) having an attached heme group. In contrast, R166K, R166M, or R166L replacements resulted in normal or near normal function. Arg-166 is conserved in all cytochromes c(1) and lies on the surface of Cyt1p in close proximity to the heme group but does not seem to interact directly with any of the physiological partners of the cytochrome bc(1) complex. Thus, the large size of the side chain at position 166 is critical for the function of cytochrome c(1) but not for its assembly in the cytochrome bc(1) complex.

Mitochondrial protein import

Most polypeptides of mitochondria are imported from the cytosol. Precursor proteins contain targeting and sorting information, often in the form of amino-terminal presequences. Precursors first bind to receptors in the outer membrane. Two putative import receptors have been identified: a 19-kilodalton protein (MOM19) in Neurospora mitochondria, and a 70-kilodalton protein (MAS70) in yeast. Some precursors integrate directly into the outer membrane, but the majority are translocated through one or both membranes. This process requires an electrochemical potential across the inner membrane. Import appears to occur through a hydrophilic pore, although the inner and outer membranes may contain functionally separate translocation machineries. In yeast, a 42-kilodalton protein (ISP42) probably forms part of the outer membrane channel. After import, precursors interact with "chaperonin" ATPases in the matrix. Presequences then are removed by the matrix protease. Finally, some proteins are retranslocated across the inner membrane to the intermembrane space.