Interaction of the precursor to mitochondrial aspartate aminotransferase and its presequence peptide with model membranes

The Functionality of Membrane-Inserting Proteins and Peptides: Curvature Sensing, Generation, and Pore Formation

Proteins and peptides with hydrophobic and amphiphilic segments are responsible for many biological functions. The sensing and generation of membrane curvature are the functions of several protein domains or motifs. While some specific membrane proteins play an essential role in controlling the curvature of distinct intracellular membranes, others participate in various cellular processes such as clathrin-mediated endocytosis, where several proteins sort themselves at the neck of the membrane bud. A few membrane-inserting proteins form nanopores that permeate selective ions and water to cross the membrane. In addition, many natural and synthetic small peptides and protein toxins disrupt the membrane by inducing nonspecific pores in the membrane. The pore formation causes cell death through the uncontrolled exchange between interior and exterior cellular contents. In this article, we discuss the insertion depth and orientation of protein/peptide helices, and their role as a sensor and inducer of membrane curvature as well as a pore former in the membrane. We anticipate that this extensive review will assist biophysicists to gain insight into curvature sensing, generation, and pore formation by membrane insertion.

Cholesterol Alters the Orientation and Activity of the Influenza Virus M2 Amphipathic Helix in the Membrane

The influenza virus M2 amphipathic helix (M2AH) alters membrane curvature in a cholesterol-dependent manner, mediating viral membrane scission during influenza virus budding. Here, we have investigated the biophysical effects of cholesterol on the ability of an M2AH peptide to manipulate membrane properties. We see that the ability of the M2AH to interact with membranes and form an α-helix is independent of membrane cholesterol concentration; however, cholesterol affects the angle of the M2AH peptide within the membrane. This change in membrane orientation affects the ability of the M2AH to alter lipid order. In low-cholesterol membranes, the M2AH is inserted near the level of the lipid head groups, increasing lipid order, which may contribute to generation of the membrane curvature. As the cholesterol content increases, the M2AH insertion becomes flatter and slightly deeper in the membrane below the lipid headgroups, where the polar face can continue to interact with the headgroups while the hydrophobic face binds cholesterol. This changed orientation minimizes lipid packing defects and lipid order changes, likely reducing the generation of membrane curvature. Thus, cholesterol regulates M2 membrane scission by precisely modulating M2AH positioning within the membrane. This has implications for the understanding of many of amphipathic-helix-driven cellular budding processes that occur in specific lipid environments.

Mitochondrial protein interaction landscape of SS-31

Mitochondrial dysfunction underlies the etiology of a broad spectrum of diseases including heart disease, cancer, neurodegenerative diseases, and the general aging process. Therapeutics that restore healthy mitochondrial function hold promise for treatment of these conditions. The synthetic tetrapeptide, elamipretide (SS-31), improves mitochondrial function, but mechanistic details of its pharmacological effects are unknown. Reportedly, SS-31 primarily interacts with the phospholipid cardiolipin in the inner mitochondrial membrane. Here we utilize chemical cross-linking with mass spectrometry to identify protein interactors of SS-31 in mitochondria. The SS-31-interacting proteins, all known cardiolipin binders, fall into two groups, those involved in ATP production through the oxidative phosphorylation pathway and those involved in 2-oxoglutarate metabolic processes. Residues cross-linked with SS-31 reveal binding regions that in many cases, are proximal to cardiolipin-protein interacting regions. These results offer a glimpse of the protein interaction landscape of SS-31 and provide mechanistic insight relevant to SS-31 mitochondrial therapy.

Electron Paramagnetic Resonance and Fluorescence Studies of the Conformation of Aspartate Aminotransferase Bound to GroEL

The interaction of the precursor to mitochondrial aspartate aminotransferase (pmAAT) with GroEL has been studied by electron paramagnetic resonance (EPR) and fluorescence spectroscopy. In the native protein, the spin probe was immobilized when attached to Cys166 at the domain interface, but was fully mobile when introduced at Cys(-19) in the N-terminal presequence peptide. Unfolding of the protein resulted in a highly mobile EPR spectrum for probes introduced at either site. However, the nitroxide group in GroEL-bound pmAAT showed either intermediate or high mobility depending on the spin probe used. Power saturation experiments indicated that the accessibility of the nitroxide side chain to Ni(EDDA) in the GroEL-pmAAT complex was higher than in the native state when in position 166 but lower when at position -19. Similar results were obtained in fluorescence quenching experiments. These data suggest that GroEL binds partly folded states of pmAAT with the presequence peptide probably in direct contact with GroEL. GroES and ATP, but not AMP-PNP or ADP, support refolding of pmAAT. During refolding, the rate of recovery of the native spectroscopic properties of labeled Cys166 is nearly identical to the rate-limiting reactivation step. Thus, correct docking of the large and small domains of pmAAT may be a key structural event in the regain of catalytic activity.

Isolation and identification of a novel mitochondrial metalloprotease (PreP) that degrades targeting presequences in plants

Most of the nuclear encoded mitochondrial precursor proteins contain an N-terminal extension called the presequence that carries targeting information and that is cleaved off after import into mitochondria. The presequences are amphiphilic, positively charged, membrane-interacting peptides with a propensity to form alpha-helices. Here we have investigated the proteolysis of the presequences that have been cleaved off inside mitochondria. A presequence derived from the overexpressed F(1)beta subunit of the ATP synthase and specific synthetic fluorescent peptides (Pep Tag Protease assay) have been shown to undergo rapid degradation catalyzed by a matrix located protease. We have developed a three-step chromatographic procedure including affinity and anion exchange chromatography for isolation of the protease from potato tuber mitochondria. Two-dimensional gel electrophoresis of the isolated proteolytically active fraction followed by electrospray ionization-mass spectrometry/mass spectrometry and data base searches allowed identification of the presequence peptide-degrading protease in Arabidopsis thaliana data base as a novel mitochondrial metalloendoprotease with a molecular mass of 105 kDa. The identified metalloprotease contains an inverted zinc-binding motif and belongs to the pitrilysin family.

Evidence that helix 8 of rhodopsin acts as a membrane-dependent conformational switch

The crystal structure of rhodopsin revealed a cytoplasmic helical segment (H8) extending from transmembrane (TM) helix seven to a pair of vicinal palmitoylated cysteine residues. We studied the structure of model peptides corresponding to H8 under a variety of conditions using steady-state fluorescence, fluorescence anisotropy, and circular dichroism spectroscopy. We find that H8 acts as a membrane-surface recognition domain, which adopts a helical structure only in the presence of membranes or membrane mimetics. The secondary structural properties of H8 further depend on membrane lipid composition with phosphatidylserine inducing helical structure. Fluorescence quenching experiments using brominated acyl chain phospholipids and vesicle leakage assays suggest that H8 lies within the membrane interfacial region where amino acid side chains can interact with phospholipid headgroups. We conclude that H8 in rhodopsin, in addition to its role in binding the G protein transducin, acts as a membrane-dependent conformational switch domain.

The solution structure and heme binding of the presequence of murine 5-aminolevulinate synthase

The mitochondrial import of 5-aminolevulinate synthase (ALAS), the first enzyme of the mammalian heme biosynthetic pathway, requires the N-terminal presequence. The 49 amino acid presequence transit peptide (psALAS) for murine erythroid ALAS was chemically synthesized, and circular dichroism and (1)H nuclear magnetic resonance (NMR) spectroscopies used to determine structural elements in trifluoroethanol/H(2)O solutions and micellar environments. A well defined amphipathic alpha-helix, spanning L22 to F33, was present in psALAS in 50% trifluoroethanol. Further, a short alpha-helix, defined by A5-L8, was also apparent in the 26 amino acid N-terminus peptide, when its structure was determined in sodium dodecyl sulfate. Heme inhibition of ALAS mitochondrial import has been reported to be mediated through cysteine residues in presequence heme regulatory motifs (HRMs). A UV/visible and (1)H NMR study of hemin and psALAS indicated that a heme-peptide interaction occurs and demonstrates, for the first time, that heme interacts with the HRMs of psALAS.