mitochondria, lipid droplets, peroxisomes - lecture 13

internal structure of mitochondria - outer mitochondrial membrane (OMM)

→ contains pores for passive diffusion of molecules up to 500 kDa

inner mitochondrial membrane (IMM) - mitochondria structure

→ single continuous membrane with three

distinct domains:

• boundary membrane (next to outer membrane)

• cristae (= ~17x plasma membrane surface area)

• crista junctions –

- sharp bends that connect the boundary

membrane to the cristae, contain MICOS

(mitochondrial contact site and cristae

organizing system)

intermembrane space - mitochondrial structure

continuous with the lumen of each crista

matrix - mitochondrial structure

→ surrounded by the inner membrane

→ citric acid cycle

→ contains mitochondrial DNA (orange spheres), ribosomes, and granules

→ F0F1 complexes regenerate ATP

mitochondria have their own genomes, mtDNA, which is packaged by TFAM:

• mtDNA is a double-stranded,

circular molecule of ~16,5 kbp and contains 37 genes coding for two rRNAs, 22 tRNAs and 13 polypeptides.

• The mtDNA-encoded polypeptides are all subunits of enzyme complexes of the oxidative phosphorylation system.

• mtDNA is packaged by TFAM

targeting of nucleus-encoded proteins to mitochondrial matrix:

→ amphipathic N-terminal targeting sequences target proteins to the mitochondrial matrix

→ N-terminal 20–50 amino acids; α-helical conformation

→ positively charged and hydrophobic amino acids predominate on opposite sides of the helix

steps of targeting nucleus-encoded proteins to mitochondrial matrix

step 1: precursor proteins synthesized on cytosolic ribosomes, maintained in an unfolded state (bound by chaperones, e.g. Hsp70)

step 2: matrix-targeting sequence (MTS) binds to outer membrane import receptor Tom20/22

step 3: MTS inserted inserted into outer membrane translocon Tom40

step 4: translocating protein moves through Tom40 and inserts into inner membrane translocon Tim (Tim17, Tim23, Tim44)

step 5:

→ protein translocates through Tim, binding by matrix Hsp70 ‘pulls’ peptide into the matrix

→ MTS removed by matrix protease

step 6: Hsp70 ATP hydrolysis releases newly imported protein

step 7: protein folds into its mature, active conformation within the matrix

the ER facilitates what?

mitochondrial constriction and Drp1-mediated fission

Drp1 (dynamin-related protein 1):

a large GTPase that mediates fission

what do fission/fusion GTPase proteins do?

balance mitochondrial morphology/activity

targeting of proteins to peroxisomes:

key functions: fatty acid oxidation synthesis of lipid produce/neutralize ROS

targeting of proteins to peroxisomes steps:

step 1:

→ peroxisomal targeting sequence PTS1 on C-terminus of proteins: S–K–L

→ PTS1 sequence binds Pex5

step 2: Pex5/PTS1-protein complex binds Pex14 receptor in peroxisomal membrane

step 3: matrix protein dissociates from Pex5, released into peroxisomal matrix (PTS1 retained)

step 4: ubiquitinylation (PTM of Lys residues with the regulatory protein ubiquitin) of Pex5 by the Pex2/10/12 complex

step 5: ATP-dependent removal of Pex5 from the membrane by ATPase proteins Pex1/Pex6

lipids droplets (LDs)

cellular organelles for neutral lipid storage

structural composition of a lipid droplet

→ monolayer-enclosed organelle with specific perilipin proteins that regulate

metabolism of neutral lipids

biogenesis of lipid droplets:

• TG synthesis within the ER (DGAT1)

• formation of an oil lens in the ER membrane (DGAT2, FIT2)

• budding and nascent lipid droplet formation (iLD, ARF1/COP1, DGAT2, GPAT4)

• lipid droplet growth and expansion via aquisition of specific proteins (eLD, CCT1)

abbreviations:

→ DGAT, diacylglycerol acyltransferase

→ eLD, expanding LD

→ iLD, initial LD

→ TG, triacylglycerol

mechanisms of targeting proteins to LD surfaces during formation and growth:

Class I proteins (e.g. GPAT4):

inserted into the ER, translocate to LDs either during formation or after via membrane bridges

Class II proteins (e.g. CCT1):

target from the cytosol via amphipathic helices or other short hydrophobic domains

lipid-droplet associated mitochondria are uniquely positioned to?

undergo beta oxidation

peri-droplet mitochondria:

• pyruvate oxidation

• ATP generation

• TAG synthesis

cytoplasmic mitochondria:

• fatty acid oxidation

• fusion-fission dynamics