Precise determination of the mitochondrial import signal contained in a 70 kDa protein of yeast mitochondrial outer membrane

Functions of outer membrane receptors in mitochondrial protein import

Most mitochondrial proteins are synthesized in the cytosol as precursor proteins and are imported into mitochondria. The targeting signals for mitochondria are encoded in the presequences or in the mature parts of the precursor proteins, and are decoded by the receptor sites in the translocator complex in the mitochondrial outer membrane. The recently determined NMR structure of the general import receptor Tom20 in a complex with a presequence peptide reveals that, although the amphiphilicity and positive charges of the presequence is essential for the import ability of the presequence, Tom20 recognizes only the amphiphilicity, but not the positive charges. This leads to a new model that different features associated with the mitochondrial targeting sequence of the precursor protein can be recognized by the mitochondrial protein import system in different steps during the import.

The first steps of protein import into mitochondria

Protein import into mitochondria is initiated by the recognition and binding of precursor proteins by import components in the cytosol, on the mitochondrial surface, and in the mitochondrial outer membrane. Following their synthesis on cytoplasmic ribosomes, some precursor proteins interact with molecular chaperones in the cytosol which function in maintaining the precursor protein in an import-competent state and may also aid in the delivery of the precursor to the mitochondria. A multisubunit protein import receptor then recognises and binds precursor proteins before feeding them into the outer membrane import site. Some proteins are sorted from the import site into the outer membrane, but most precursor proteins travel through the outer membrane import site into the mitochondria, where the later steps of protein import take place.

Mitochondrial receptor complex protein. The intermembrane space domain of yeast MAS17 is not essential for its targeting or function

MAS17 (MAS22) is an essential component of the import receptor complex in the yeast mitochondrial outer membrane. MAS17 consists of three distinct domains: the N-terminal cytosolic domain, the internal membrane-spanning domain, and the C-terminal intermembrane space domain. In the present study, we examined the roles of the C-terminal domain of MAS17, which is rich in acidic amino acids, in protein import into mitochondria both in vivo and in vitro. Cells expressing MAS17 delta 120-152, a mutant MAS17 lacking the C-terminal acidic domain, could grow as fast as those expressing wild-type MAS17, while cells expressing MAS17 delta 97-152, a mutant MAS17 lacking both the intermembrane space and the membrane-spanning domains, stopped growing as soon as wild-type MAS17 was depleted. MAS17 delta 120-152 was correctly integrated into the mitochondrial outer membrane like wild-type MAS17. Mitochondria containing MAS17 delta 120-152 instead of wild-type MAS17 could import both authentic and artificial mitochondrial precursor proteins nearly as efficiently as wild-type mitochondria in vitro. These results suggest that the C-terminal intermembrane space domain of MAS17 is not essential for targeting or functions of MAS17.

Insertion of an uncharged polypeptide into the mitochondrial inner membrane does not require a trans-bilayer electrochemical potential: effects of positive charges

Mitochondria with a ruptured outer membrane exhibited impaired import into this membrane of an outer membrane fusion protein containing the signal-anchor sequence of Mas70p. However, the Mas70p signal-anchor efficiently targeted and inserted the protein directly into exposed regions of the inner membrane. Import into the inner membrane was dependent on delta psi and this dependence was due to the presence of the positively-charged amino acids located at positions 2, 7, and 9 of the signal-anchor. In contrast to wild-type signal-anchor, mutants lacking the positively-charged residues mediated import into the inner membrane in both the presence and absence of delta psi. The results suggest two conclusions: (1) delta psi-dependent import of the signal-anchor sequence was due exclusively to an effect of delta psi on the positively-charged domain of the signal-anchor, rather than to an effect of delta psi on a property of the inner membrane import machinery; (2) in the absence of delta psi, the positively-charged domain of the signal-anchor prevented the otherwise import-competent signal-anchor from inserting into the membrane. This suggests that the positively-charged domain leads import across the inner membrane, and that delta psi is required to vectorially clear this domain in order to allow the distal region of the signal-anchor to enter the translocation pathway. The implications of these findings on the mechanism of import into the mitochondrial inner membrane and matrix are discussed.

Import and insertion of proteins into the mitochondrial outer membrane

Nuclear-encoded proteins destined for insertion into the mitochondrial outer membrane, follow the same general pathway for import as proteins that are translocated to interior compartments within the organelle. This observation is true both for beta-barrel-type proteins and for proteins that contain hydrophobic alpha-helical transmembrane segments. In this review, we describe what is known about the various steps leading to protein insertion into the outer membrane, and discuss the energetics that favor vectorial translocation into and across this membrane. The selection of the outer membrane during import may involve a lateral release of the translocating polypeptide from the import machinery so that the appropriate domains of the protein become embedded in the lipid bilayer. One type of topogenic domain that can guarantee such selection of the outer membrane is a signal-anchor sequence of the type characterized for the bitopic protein Mas70p. It is suggested that a signal-anchor sequence selective for the mitochondrial outer membrane causes abrogation of polypeptide translocation and triggers the release of the transmembrane segment into the surrounding lipid bilayer, prior to any possibility for the commitment of translocation to the interior of the organelle. Specific structural features of the signal-anchor sequence specify its orientation in the membrane, and can confer on this sequence the ability to form homo-oligomers and hetero-oligomers. Strategies other than a signal-anchor sequence may be employed by other classes of proteins for selection of the outer-membrane. Of note is the ability of the outer-membrane import machinery to catalyze integration of the correct set of proteins into the outer-membrane bilayer, while allowing proteins that are destined for integration into the bilayer of the inner membrane to pass through unimpeded. Again, however, different proteins may employ different strategies. One model proposes that this can be accomplished by a combination of a matrix-targeting signal and a distal stop-transfer sequence. In this model, the formation of contact sites, which is triggered when the matrix-targeting signal engages the import machinery of the inner membrane, may prevent the outer-membrane translocon from recognizing and responding to the downstream stop-transfer domain. This allows the transmembrane segment to pass across the outer-membrane, and subsequently integrate into the inner membrane.

A signal-anchor sequence selective for the mitochondrial outer membrane

pOMD29 is a hybrid protein containing the NH2-terminal topogenic sequence of a bitopic, integral protein of the outer mitochondrial membrane in yeast, OMM70, fused to dihydrofolate reductase. The topogenic sequence consists of two structural domains: an NH2-terminal basic region (amino acids 1-10) and an apolar region which is the predicted transmembrane segment (amino acids 11-29). The transmembrane segment alone was capable of targeting and inserting the hybrid protein into the outer membrane of intact mitochondria from rat heart in vitro. The presence of amino acids 1-10 enhanced the rate of import, and this increased rate depended, in part, on the basic amino acids located at positions 2, 7, and 9. Deletion of a large portion of the transmembrane segment (amino acids 16-29) resulted in a protein that exhibited negligible import in vitro. Insertion of pOMD29 into the outer membrane was not competed by import of excess precursor protein destined for the mitochondrial matrix, indicating that the two proteins may have different rate-limiting steps during import. We propose that the structural domains within amino acids 1-29 of pOMD29 cooperate to form a signal-anchor sequence, the characteristics of which suggest a model for proper sorting to the mitochondrial outer membrane.

Targeting proteins to mitochondria: a current overview

Images

Protein sorting between mitochondrial outer and inner membranes. Insertion of an outer membrane protein into the inner membrane

The amino terminal 29 amino acids of the outer mitochondrial membrane protein of yeast, OMM70 (MAS70), consisting of the targeting and membrane anchor domains, has been fused to a reporter protein, dihydrofolate reductase. The hybrid protein, designated pOMD29, was efficiently imported into the outer membrane of rat heart mitochondria by a process dependent on ATP and proteinase-sensitive components on the surface of the organelle, and in which the orientation of the native protein was retained. To determine if the protein translocation machinery of the inner membrane is also capable of recognizing and inserting pOMD29, direct access to the intermembrane space was provided to pOMD29 by selectively rupturing the mitochondrial outer membrane by osmotic shock. In this system, the outer membrane binding site for matrix-destined precursor proteins can be bypassed, and efficient import restored to proteinase-pretreated mitochondria. pOMD29 was imported into the inner membrane of osmotically-shocked mitochondria, mediated by protein components. The outer membrane orientation of pOMD29 was conserved when inserted into the inner membrane but, unlike the outer membrane, import into the inner membrane required delta psi. We conclude that the protein translocation machinery of the mitochondrial inner membrane is capable of recognizing and inserting a protein whose topogenic information otherwise results in insertion of the protein to the outer membrane. The significance of these findings for sorting of proteins between the mitochondrial inner and outer membranes is discussed.

PAS3, a Saccharomyces cerevisiae gene encoding a peroxisomal integral membrane protein essential for peroxisome biogenesis

Saccharomyces cerevisiae pas3-mutants are described which conform the pas-phenotype recently reported for the peroxisomal assembly mutants pas1-1 and pas2 (Erdmann, R., M. Veenhuis, D. Mertens, and W.-H Kunau, 1989, Proc. Natl. Acad. Sci. USA. 86:5419-5423). The isolation of pas3-mutants enabled us to clone the PAS3 gene by functional complementation. DNA sequence analysis revealed a 50.6-kD protein with at least one domain of sufficient length and hydrophobicity to span a lipid bilayer. To verify these predictions antibodies were raised against a truncated portion of the PAS3 coding region overexpressed in E. coli. Pas3p was identified as a 48 kD peroxisomal integral membrane protein. It is shown that a lack of this protein causes the peroxisome-deficient phenotype and the cytosolic mislocalization of peroxisomal matrix enzymes. Based on protease digestion experiments Pas3p is discussed to be anchored in the peroxisomal membrane by its amino-terminus while the bulk of the molecule is exposed to the cytosol. These findings are consistent with the possibility that Pas3p is one component of the peroxisomal import machinery.

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.

Import of ADP/ATP carrier into mitochondria: two receptors act in parallel

We have identified the yeast homologue of Neurospora crassa MOM72, the mitochondrial import receptor for the ADP/ATP carrier (AAC), by functional studies and by cDNA sequencing. Mitochondria of a yeast mutant in which the gene for MOM72 was disrupted were impaired in specific binding and import of AAC. Unexpectedly, we found a residual, yet significant import of AAC into mitochondria lacking MOM72 that occurred via the receptor MOM19. We conclude that both MOM72 and MOM19 can direct AAC into mitochondria, albeit with different efficiency. Moreover, the precursor of MOM72 apparently does not require a positively charged sequence at the extreme amino terminus for targeting to mitochondria.

Embedded or not? Hydrophobic sequences and membranes

The same translocation machinery appears to be responsible for both the translocation of soluble proteins across membranes and the insertion of integral membrane proteins into the bilayer. A single mechanism is proposed to accommodate these two functions. This model is also extended to explain the paradoxical translocation of mitochondrial and chloroplastic membrane proteins across one or more membranes before they are finally inserted into their target membranes.