Lecture 17-Targeting proteins to Peroxisome and Nucleus

Single membrane bound oxidative organelles

Peroxisome

Contains enzymes (oxidases) that use molecular oxygen to oxidize various substrates such as fatty acids, breaking them down into smaller components to use in biosynthetic pathways

Peroxisome

Peroxisome Generates

hydrogen peroxide (H2O2)

enzyme catalase that converts H2O2 to

oxygen and water

Peroxisome are most abundant where

liver cells

Switches from monomeric (composed of a single unit, such as a protein with one polypeptide chain or a small molecule that can form a polymer) soluble form to an oligomeric (molecule composed of a small number of repeating structural units, known as monomers, which are linked by covalent bonds) form embedded in peroxisomal membrane forming a complex with membrane protein Pex14

Pex5

Unlike the ER and Mitochondria, peroxisomes can import what 

folded proteins

one of a group of four related diseases called peroxisome biogenesis disorders

Zellweger Syndrome

genetic material and associated protein (histones)

Chromatin

ribosomal RNA production and assembly of ribosome components

Nucleolus

non-nucleolar regions of the nucleus

Nucleoplasm

lamin filaments and associated proteins

Form scaffolding/structural support

Nuclear matrix

What is connected to the ER

Nuclear outer membrae

What perforate the nuclear envelope

Nuclear Pores

Nuclear pores are composed multi-protein structure
called

Nuclear Pore Complex

How many nuclear membranes does the Nuclear Pore Complex span

Both

Which size molecules and proteins can diffuse through passively (<40 kDa)

Small

Which size proteins need to be actively transported with the help of soluble transport proteins that interacts with the cargo and components of the nuclear pore

Large

Membrane embedded ring-structure with an aqueous pore

Cytoplasmic face

• Distal Ends of the filaments joined by a terminal ring forming a nuclear basket structure

Nucleoplasmic face

Forms the scaffold of the nuclear pore
Forms an octagonal ring
Spans both inner and outer membranes

Seven structural nucleoporins form a Y- shaped structure known as the Y-complex

Sixteen copies of the Y-complex form the basic structural scaffold of the pore

Structural nucleoporins

The inner and outer membranes of the nuclear envelope are connected at the NPC by a highly curved region of membrane

This region contains the embedded membrane nucleoporins

Membrane nucleoporins associate with both the scaffold (structural nucleoporins) and the nuclear membrane

Membrane Nucleoporins

Contain multiple repeats of short hydrophobic sequences rich in phenylalanine (F) and glycine (G) residues

Line the channel of the NPC

Also found associated with the nuclear basket and cytoplasmic filaments

Forms a gel-like matrix

-Allows small molecules to diffuse

Blocks unchaperoned translocation

of proteins >40 kDa

Intrinsically disordered protein

FG-Nucleoporins

Targets protein for import through nuclear pores into nucleus

Stretch of basic amino acids (e.g., Pro-Lys-Lys-Lys-Arg-Lys-Val)

Nuclear localization signal

Located: N-terminus

6-12 hydrophobic residues,1 basic residue

Leucine, Isoleucine, Valine, Phenylalanine, Methionine, Alanine, Tryptophan, 

Lysine, Arginine, Histidine

ER

Located: N-terminus

3-5 (non-consecutive) Arg, Lys

Mitochondria

Located: N-terminus

Ser & Thr rich

Chloroplast

Located: C-terminus

Ser-Lys-Leu

Peroxisome

Located: Internal

Leucine, Isoleucine, Valine, Phenylalanine, Methionine, Alanine, Tryptophan,

Lysine, Arginine, Histidine

Nucleus

Cytosolic components required for Nuclear-localization signal (NLS)

Nuclear transport receptors (importin) and RanGTPase

Binds to NLS on the cargo protein
Has affinity for FG-repeats on nucleoporins
By binding to cargo protein and interacting with FG-nucleoporins they facilitate the transport of cargo across the nuclear pore

Nuclear transport receptors (importin)

Small monomeric G-protein

Exists in either a GTP-bound or GDP-bound confirmation

Cycling of Ran between GTP-bound and GDP-bound conformations by hydrolysis of GTP→GDP provides energy to drive unidirectional transport through the nuclear pore

Ran GTPase

found in PKI (an inhibitor of Protein Kinase A) and Rev protein in HIV

Leucine Rich sequence

Precise structural features that determine recognition of these NES remain poorly understood

Heterogenous ribonucleoproteins

Binds to NES on the cargo protein

Has affinity for FG-repeats on nucleoporins

By binding to cargo protein and interacting with FG-nucleoporins they facilitate the transport of cargo across the nuclear pore

Nuclear Transport Receptor (Exportin 1)

In the nucleus, mRNA is associated with specific proteins and form a complex known as

Messenger Ribonuclear Protein Complex (mRNP)

mRNP is exported from the nucleus with the help of

mRNP Exporter

Heterodimeric protein

Large subunit called nuclear export factor 1 (NXF1)

Small subunit called nuclear export transporter 1 (NXT1)

Multiple NXF1/NXT1 dimers bind to mRNP

mRNP Exporter