Quiz 4 | Chapter 12 | Intracellular Organization and Protein Sorting

Outer mitochondrial membrane

Highly porous due to porins (holes allowing passive transport of molecules up to ~5 kDa). Similar feature found on gram-negative bacteria

Inner mitochondrial matrix

Highly folded into cristae to increase surface area (involves ~7570 proteins)

Citric acid cycle

Metabolic process that occurs in the mitochondrial matrix

Signal sequence for entering mitochondrial matrix

Located at the N-terminus (beginning); an alpha helix containing positively charged amino acids

Polypeptide import into mitochondrial matrix

• Synthesized in cytosol and bound by Hsp70 chaperones (kept unfolded).

• Complex binds to receptor proteins on the outer membrane (TOM complex).

• Polypeptide is inserted through the TOM channel (aided by chaperones using ATP hydrolysis).

• Passes through the inner membrane via the TIM23 complex (driven by the positively charged IMS repelling the signal sequence, attraction to the negative matrix, and Hsp70 "walking" via ATP).

• Signal sequence is cleaved off and the protein folds.

Mitochondrial outer membrane protein insertion

MIM complex works with TOM or SAM (sorting & assembly machinery; homologous to BAM in bacteria)

Mitochondrial intermembraneous space protein insertion

Mia-40 adds disulfide bonds to prevent threading back out through TOM

Mitochondrial inner membrane protein insertion

Uses TIM23 or TIM22 via a lateral gate, or OXA (oxidase assembly) for polypeptides synthesized directly in the matrix from the mitochondrial genome

Chloroplast transport

TOCS (transporters of outer chloroplast) and TICS (transporters of inner chloroplast)

Peroxisomes

Single-membrane organelles full of enzymes

Peroxisomal beta-oxidation (breaks down fatty acids, producing H2O2 which catalase converts to 2H2O + O2; synthesis of cholesterol, plasmalogens (makes up 70% of myelin sheath), and bile salts

Divide via binary fission; signal sequence at C-terminus binds peroxin proteins for insertion (plus some vesicular transport)

Nuclear envelope

Double membrane (outer membrane is continuous with the ER and studded with ribosomes; IMS is continuous with the ER lumen). Inner membrane faces the nucleoplasm and is lined with the nuclear lamina (composed of intermediate filament proteins called lamins) which provides shape, support, and organizes DNA

Nuclear pore complex

Octagonal shape made of ~500–1000 proteins (~30 different proteins called nucleoporins). Small molecules/ions (<5 kDa) pass via simple diffusion; anything larger requires escorted transport using FG repeats (phenylalanine and glycine) on the fibrils

Nuclear Import

• Fully folded proteins contain a Nuclear Localization Signal (NLS) (string of positive amino acids).

• Importin receptors bind the NLS and interact with FG repeats to move down the concentration gradient into the nucleus.

• Inside, Ran-GTP binds importin, causing it to release the cargo.

• Ran-GTP-importin returns to the cytosol, where GTP is hydrolyzed (by Ran-GAP) to release the importin for recycling.

Nuclear export

• Proteins with a Nuclear Export Signal bind to exportins only when exportins are already bound to Ran-GTP in the nucleus.

• The complex moves out through the pore into the cytosol.

• Ran-GTP is hydrolyzed to Ran-GDP, causing the exportin to change conformation and release the cargo.

Breakdown of nuclear envelope

During mitosis/meiosis, lamins are phosphorylated (adding negative charges), causing the nuclear lamina to fall apart. At the end of division, they are dephosphorylated so the membrane can reform