Targeting and insertion of C-terminally anchored proteins to the mitochondrial outer membrane is specific and saturable but does not strictly require ATP or molecular chaperones

Positively charged residues within the MYO19 MyMOMA domain are essential for proper localization of MYO19 to the mitochondrial outer membrane

Myosins are well characterized molecular motors essential for intracellular transport. MYO19 copurifies with mitochondria, and can be released from mitochondrial membranes by high pH buffer, suggesting that positively-charged residues participate in interactions between MYO19 and mitochondria. The MYO19-specific mitochondria outer membrane association (MyMOMA) domain contains approximately 150 amino acids with a pI approximately 9 and is sufficient for localization to the mitochondrial outer membrane. The minimal sequence and specific residues involved in mitochondrial binding have not been identified. To address this, we generated GFP-MyMOMA truncations, establishing the boundaries for truncations based on sequence homology. We identified an 83-amino acid minimal binding region enriched with basic residues (pI ∼ 10.5). We sequentially replaced basic residues in this region with alanine, identifying residues R882 and K883 as essential for mitochondrial localization. Constructs containing the RK882-883AA mutation primarily localized with the endoplasmic reticulum (ER). To determine if ER-associated mutant MyMOMA domain and mitochondria-associated wild type MyMOMA display differences in kinetics of membrane interaction, we paired FRAP analysis with permeabilization activated reduction in fluorescence (PARF) analysis. Mitochondria-bound and ER-bound MYO19 constructs displayed slow dissociation from their target membrane when assayed by PARF; both constructs displayed exchange within their respective organelle networks. However, ER-bound mutant MYO19 displayed more rapid exchange within the ER network than did mitochondria-bound MYO19. Taken together these data indicate that the MyMOMA domain contains strong membrane-binding activity, and membrane targeting is mediated by a specific, basic region of the MYO19 tail with slow dissociation kinetics appropriate for its role(s) in mitochondrial network dynamics. © 2016 Wiley Periodicals, Inc.

Adaptor proteins MiD49 and MiD51 can act independently of Mff and Fis1 in Drp1 recruitment and are specific for mitochondrial fission

Drp1 (dynamin-related protein 1) is recruited to both mitochondrial and peroxisomal membranes to execute fission. Fis1 and Mff are Drp1 receptor/effector proteins of mitochondria and peroxisomes. Recently, MiD49 and MiD51 were also shown to recruit Drp1 to the mitochondrial surface; however, different reports have ascribed opposing roles in fission and fusion. Here, we show that MiD49 or MiD51 overexpression blocked fission by acting in a dominant-negative manner by sequestering Drp1 specifically at mitochondria, causing unopposed fusion events at mitochondria along with elongation of peroxisomes. Mitochondrial elongation caused by MiD49/51 overexpression required the action of fusion mediators mitofusins 1 and 2. Furthermore, at low level overexpression when MiD49 and MiD51 form discrete foci at mitochondria, mitochondrial fission events still occurred. Unlike Fis1 and Mff, MiD49 and MiD51 were not targeted to the peroxisomal surface, suggesting that they specifically act to facilitate Drp1-directed fission at mitochondria. Moreover, when MiD49 or MiD51 was targeted to the surface of peroxisomes or lysosomes, Drp1 was specifically recruited to these organelles. Moreover, the Drp1 recruitment activity of MiD49/51 appeared stronger than that of Mff or Fis1. We conclude that MiD49 and MiD51 can act independently of Mff and Fis1 in Drp1 recruitment and suggest that they provide specificity to the division of mitochondria.

Targeting and insertion of the cholesterol-binding translocator protein into the outer mitochondrial membrane

Translocator protein (18 kDa, TSPO), previously known as the peripheral-type benzodiazepine receptor, is an outer mitochondrial membrane (OMM) protein necessary for cholesterol import and steroid production. We reconstituted the mitochondrial targeting and insertion of TSPO into the OMM to analyze the signals and mechanisms required for this process. Initial studies indicated the formation of a mitochondrial 66 kDa complex through Blue Native-PAGE analysis. The formation of this complex was found to be dependent on the presence of ATP and the cytosolic chaperone Hsp90. Through mutational analysis we identified two areas necessary for TSPO targeting, import, and function: amino acids 103-108 (Schellman motif), which provide the necessary structural orientation for import, and the cholesterol-binding C-terminus required for insertion. Although the translocase of the outer mitochondrial membrane (TOM) complex proteins Tom22 and Tom40 were present in the OMM, the TOM complex did not interact with TSPO. In search of proteins involved in TSPO import, we analyzed complexes known to interact with TSPO by mass spectrometry. Formation of the 66 kDa complex was found to be dependent on an identified protein, Metaxin 1, for formation and TSPO import. The level of import of TSPO into steroidogenic cell mitochondria was increased following treatment of the cells with cAMP. These findings suggest that the initial targeting of TSPO to mitochondria is dependent upon the presence of cytosolic chaperones interacting with the import receptor Tom70. The C-terminus plays an important role in targeting TSPO to mitochondria, whereas its import into the OMM is dependent upon the presence of the Schellman motif. Final integration of TSPO into the OMM occurs via its interaction with Metaxin 1. Import of TSPO into steroidogenic cell mitochondria is regulated by cAMP.

Biogenesis of mitochondrial outer membrane proteins

Mitochondria are surrounded by two distinct membranes: the outer and the inner membrane. The mitochondrial outer membrane mediates numerous interactions between the mitochondrial metabolic and genetic systems and the rest of the eukaryotic cell. Proteins of this membrane are nuclear-encoded and synthesized as precursor proteins in the cytosol. They are targeted to the mitochondria and inserted into their target membrane via various pathways. This review summarizes our current knowledge of the sorting signals for this specific targeting and describes the mechanisms by which the mitochondrial import machineries recognize precursor proteins, mediate their membrane integration and facilitate assembly into functional complexes.

The C-terminus of cytochrome b5 confers endoplasmic reticulum specificity by preventing spontaneous insertion into membranes

The molecular mechanisms that determine the correct subcellular localization of proteins targeted to membranes by tail-anchor sequences are poorly defined. Previously, we showed that two isoforms of the tung oil tree [Vernicia (Aleurites) fordii] tail-anchored Cb5 (cytochrome b5) target specifically to ER (endoplasmic reticulum) membranes both in vivo and in vitro [Hwang, Pelitire, Henderson, Andrews, Dyer and Mullen (2004) Plant Cell 16, 3002-3019]. In the present study, we examine the targeting of various tung Cb5 fusion proteins and truncation mutants to purified intracellular membranes in vitro in order to assess the importance of the charged CTS (C-terminal sequence) in targeting to specific membranes. Removal of the CTS from tung Cb5 proteins resulted in efficient binding to both ER and mitochondria. Results from organelle competition, liposome-binding and membrane proteolysis experiments demonstrated that removal of the CTS results in spontaneous insertion of tung Cb5 proteins into lipid bilayers. Our results indicate that the CTSs from plant Cb5 proteins provide ER specificity by preventing spontaneous insertion into incorrect subcellular membranes.

Mitochondrial protein import and human health and disease

The targeting and assembly of nuclear-encoded mitochondrial proteins are essential processes because the energy supply of humans is dependent upon the proper functioning of mitochondria. Defective import of mitochondrial proteins can arise from mutations in the targeting signals within precursor proteins, from mutations that disrupt the proper functioning of the import machinery, or from deficiencies in the chaperones involved in the proper folding and assembly of proteins once they are imported. Defects in these steps of import have been shown to lead to oxidative stress, neurodegenerative diseases, and metabolic disorders. In addition, protein import into mitochondria has been found to be a dynamically regulated process that varies in response to conditions such as oxidative stress, aging, drug treatment, and exercise. This review focuses on how mitochondrial protein import affects human health and disease.

Divergent mitochondrial and endoplasmic reticulum association of DMPK splice isoforms depends on unique sequence arrangements in tail anchors

Myotonic dystrophy protein kinase (DMPK) is a Ser/Thr-type protein kinase with unknown function, originally identified as the product of the gene that is mutated by triplet repeat expansion in patients with myotonic dystrophy type 1 (DM1). Alternative splicing of DMPK transcripts results in multiple protein isoforms carrying distinct C termini. Here, we demonstrate by expressing individual DMPKs in various cell types, including C(2)C(12) and DMPK(-/-) myoblast cells, that unique sequence arrangements in these tails control the specificity of anchoring into intracellular membranes. Mouse DMPK A and C were found to associate specifically with either the endoplasmic reticulum (ER) or the mitochondrial outer membrane, whereas the corresponding human DMPK A and C proteins both localized to mitochondria. Expression of mouse and human DMPK A-but not C-isoforms in mammalian cells caused clustering of ER or mitochondria. Membrane association of DMPK isoforms was resistant to alkaline conditions, and mutagenesis analysis showed that proper anchoring was differentially dependent on basic residues flanking putative transmembrane domains, demonstrating that DMPK tails form unique tail anchors. This work identifies DMPK as the first kinase in the class of tail-anchored proteins, with a possible role in organelle distribution and dynamics.

Signal recognition particle mediates post-translational targeting in eukaryotes

Signal recognition particle (SRP) plays a central role in the delivery of classical secretory and membrane proteins to the endoplasmic reticulum (ER). All nascent chains studied to date dissociate from SRP once released from the ribosome, thereby supporting a strictly cotranslational mode of action for eukaryotic SRP. We now report a novel post-translational function for SRP in the targeting of tail-anchored (TA) proteins to the ER. TA proteins possess a hydrophobic membrane insertion sequence at their C-terminus such that it can only emerge from the ribosome after translation is terminated. We show that SRP can associate post-translationally with this type of ER-targeting signal, and deliver newly synthesised TA proteins to the ER membrane by a pathway dependent upon GTP and the SRP receptor. We find that dependency upon this SRP-dependent route is precursor specific, and propose a unifying model to describe the biogenesis of TA proteins in vivo.

Finding the right organelle. Targeting signals in mitochondrial outer-membrane proteins

The mitochondrial outer membrane contains a diverse set of proteins that includes enzymes, components of the preprotein translocation machinery, pore-forming proteins, regulators of programmed cell death, and those that control the morphology of the organelle. All these proteins, like the vast majority of mitochondrial proteins, are encoded in the nucleus, so they are synthesized in the cytosol and contain signals that are essential for their subsequent import into mitochondria. This review summarizes our current knowledge of the signals that target mitochondrial outer-membrane proteins to their correct intracellular location. In addition, the mechanisms by which these signals are decoded by the mitochondria are discussed.

Bcl-2 family members: integrators of survival and death signals in physiology and pathology [corrected]

The members of the Bcl-2 family of proteins are crucial regulators of apoptosis. In order to determine cell fate, these proteins must be targeted to distinct intracellular membranes, including the mitochondrial outer membrane (MOM), the membrane of the endoplasmic reticulum (ER) and its associated nuclear envelope. The targeting sequences and mechanisms that mediate the specificity of these proteins for a particular cellular membrane remain poorly defined. Several Bcl-2 family members have been reported to be tail-anchored via their predicted hydrophobic COOH-terminal transmembrane domains (TMDs). Tail-anchoring imposes a posttranslational mechanism of membrane insertion on the already folded protein, suggesting that the transient binding of cytosolic chaperone proteins to the hydrophobic TMD may be an important regulatory event in the targeting process. The TMD of certain family members is initially concealed and only becomes available for targeting and membrane insertion in response to apoptotic stimuli. These proteins either undergo a conformational change, posttranslational modification or a combination of these events enabling them to translocate to sites at which they are functional. Some Bcl-2 family members lack a TMD, but nevertheless localize to the MOM or the ER membrane during apoptosis where they execute their functions. In this review, we will focus on the intracellular targeting of Bcl-2 family members and the mechanisms by which they translocate to their sites of action. Furthermore, we will discuss the posttranslational modifications which regulate these events.

The tale of tail-anchored proteins: coming from the cytosol and looking for a membrane

A group of integral membrane proteins, known as C-tail anchored, is defined by the presence of a cytosolic NH2-terminal domain that is anchored to the phospholipid bilayer by a single segment of hydrophobic amino acids close to the COOH terminus. The mode of insertion into membranes of these proteins, many of which play key roles in fundamental intracellular processes, is obligatorily posttranslational, is highly specific, and may be subject to regulatory processes that modulate the protein's function. Although recent work has elucidated structural features in the tail region that determine selection of the correct target membrane, the molecular machinery involved in interpreting this information, and in modulating tail-anchored protein localization, has not been identified yet.

Characterization of the signal that directs Bcl-x(L), but not Bcl-2, to the mitochondrial outer membrane

It is assumed that the survival factors Bcl-2 and Bcl-x(L) are mainly functional on mitochondria and therefore must contain mitochondrial targeting sequences. Here we show, however, that only Bcl-x(L) is specifically targeted to the mitochondrial outer membrane (MOM) whereas Bcl-2 distributes on several intracellular membranes. Mitochondrial targeting of Bcl-x(L) requires the COOH-terminal transmembrane (TM) domain flanked at both ends by at least two basic amino acids. This sequence is a bona fide targeting signal for the MOM as it confers specific mitochondrial localization to soluble EGFP. The signal is present in numerous proteins known to be directed to the MOM. Bcl-2 lacks the signal and therefore localizes to several intracellular membranes. The COOH-terminal region of Bcl-2 can be converted into a targeting signal for the MOM by increasing the basicity surrounding its TM. These data define a new targeting sequence for the MOM and propose that Bcl-2 acts on several intracellular membranes whereas Bcl-x(L) specifically functions on the MOM.

Biogenesis of the mitochondrial TOM complex

The translocase at the outer membrane of mitochondria (TOM complex) mediates the initial steps of the import of preproteins into the organelle, which are essential for mitochondrial biogenesis and, therefore, for eukaryotic cell viability. The TOM complex is a multisubunit molecular machine with a dynamic structure. The biogenesis of TOM is of special interest because the complex is required for its own biogenesis. This article describes the mechanisms by which Tom components are targeted to the mitochondria and inserted into the outer membrane. The assembly of newly synthesized subunits into the functional TOM complex might occur via assembly intermediates that are in equilibrium with the mature complex.

Targeting of a tail-anchored protein to endoplasmic reticulum and mitochondrial outer membrane by independent but competing pathways

Many mitochondrial outer membrane (MOM) proteins have a transmembrane domain near the C terminus and an N-terminal cytosolic moiety. It is not clear how these tail-anchored (TA) proteins posttranslationally select their target, but C-terminal charged residues play an important role. To investigate how discrimination between MOM and endoplasmic reticulum (ER) occurs, we used mammalian cytochrome b(5), a TA protein existing in two, MOM or ER localized, versions. Substitution of the seven C-terminal residues of the ER isoform or of green fluorescent protein reporter constructs with one or two arginines resulted in MOM-targeted proteins, whereas a single C-terminal threonine caused promiscuous localization. To investigate whether targeting to MOM occurs from the cytosol or after transit through the ER, we tagged a MOM-directed construct with a C-terminal N-glycosylation sequence. Although in vitro this construct was efficiently glycosylated by microsomes, the protein expressed in vivo localized almost exclusively to MOM, and was nearly completely unglycosylated. The small fraction of glycosylated protein was in the ER and was not a precursor to the unglycosylated form. Thus, targeting occurs directly from the cytosol. Moreover, ER and MOM compete for the same polypeptide, explaining the dual localization of some TA proteins.