Chapters 12 & 15

Chapter 12: Transport
Across Membranes &
Neurons

Transporters & Channels

●Things to note:

○Cells are closed systems and carefully regulate what comes in and out: a point of regulation

○Transporter:

■Allows passage of molecules or ions one at a time by changing the conformation of the channel (great specificity & very selective)

○Channel:

■Discriminate based on size and electric charge

■Like a gate

●Types of Transport:

○Passive:

■“Downhill” flow → high to low concentration

■No other driving force needed because it happens naturally

○Active:

■“Uphill” flow → low to high concentration

■Goes AGAINST CONCENTRATION so it needs an energy input

Ca+ Pump

Ion Channels

●Channels are selective

●Gated-ion channels

○NOT continuously open

○GATED ion channels respond to different types of stimuli

■Membrane Potential

●Voltage gated channels respond to membrane potential
- there are voltage sensors that are sensitive to changes in membrane potential

■Ligand Binding Channel

●A ligand can bind extracellularly or intracellularly to control the opening
of the channel

●Example → Neurotransmitter-gated ion channel

Chapter 15

Membrane Enclosed Organelles

Membrane Enclosed Organelles

○Through the invagination of the plasma membrane, forming a two-layer envelope around genetic material
- invagination = the process of being folded back on itself to create a cavity

○Signal sequences: conserved amino acid sequence that acts as a molecular “address” telling the protein where the protein lives in the cell

○Without these localization sequences, protein will not reach its designated destination

●How can proteins cross the phospholipid bilayer?

1. Cytosol -> nucleus

   1. nuclear pores

2. Cytosol -> ER, mitochondria, chloroplast

   1. protein translocators in membrane

3. ER onward and from one compartment of endomembrane systems into another

   1. transport vesicles

How can proteins cross phospholipid bilayer?

1.Cytosol into the Nucleus

- via Nuclear pores (penetrate the inner and outer membranes)

2.Cytosol into ER, Mitochondria,
chloroplast

-Protein translocators in
membrane also similar to bacteria

3.ER onward and from one
compartment of endomembrane
system to another via -Transport
vesicles

Protein Transport

●How do proteins enter the nucleus?

○Through the nuclear pores: form the gate through which all molecules enter or leave the nucleus

○Nuclear transport receptors (NTRs): grab onto repeated amino acid sequences
within the tangle of the nuclear pore proteins to carry the protein cargo into the
nucleus

■Requires energy! (GTP)

■Nuclear pores transport proteins in their fully folded conformation

The Nucleus

•Inner & Outer nuclear membrane
•Nuclear lamina - protein filaments
that provide structural support for
the nuclear envelope
•Nuclear pores - form the gates
which all molecules enter or leave
the nucleus

Nuclear transport receptors actively transport proteins through nuclear pores

-NTR- Grabs onto repeated AA
sequence of nuclear pore proteins to carry the protein cargo into the
nucleus which requires GTP energy

-Nuclear pores transport proteins in their fully folded conformation

Protein Transport-Mitochondria, Chloroplasts

●Proteins have to unfold to enter these
endosymbionts

●Translocator channels: big
transmembrane proteins that take
proteins and transport them through
the membrane so that they can get to
the other side of the membrane

○Protein translocator recognizes signal sequence

○Two sets of these translocators on the outer and inner mitochondrial matrix

○After translocation is complete,
signal sequence is cleaved and
protein refolds

Proteins enter the Mitochondria and chloroplasts

-Proteins MUST UNFOLD in order to
enter the mitochondria and
chloroplasts

-Proteins that enter must have an N-TERMINAL signal (red)

-Proteins translocate across both membranes simultaneously at
specific sites

-Proteins unfolded as they are
transported

-Signal sequence removed after
translocation completed and protein refolds

Protein Transport-Endoplasmic Reticulum

●Endoplasmic Reticulum (ER): the highway of the cell; most extensive system in the
eukaryotic cell

○Entry point for proteins destined for other organelles as well as the ER itself

●Proteins that enter the ER begin entering the ER membrane before the polypeptide chain has been fully synthesized

○As soon as the ER signal sequence is translated into protein, that will be recognized
by the ER, and the protein will immediately start feeding into the ER

●Translocation channels help soluble proteins cross the ER to enter the lumen

○ER signal sequence (on N-terminus) functions to open channel

○Remains bound to channel as remainder of protein chain is threaded through membrane as a large loop

○ER signal cleaved after proteins have crossed

○Once protein is inside the lumen, the protein folds into its final structure

Protein Transport -Beyond ER

●Once a protein enters the ER, it never returns
to the cytosol and travels by vesicle

●Vesicles are formed using a protein coat
(scooping up part of membrane and making a bubble)

○Protein coat: shapes membrane into a bud, helps capture molecules for onward transport

Clathrin proteins help make these bubbles: Has a triskelion structure, causing the formation of a circular structure out of necessity at they fit together

■Once the vesicle has fully
formed, the clathrin will fall off

●How are vesicles transported?

○Actively transported by motor
(walking) proteins the move along the cytoskeleton (Ch. 17)

Protein Transport- Beyond ER

●Vesicle reaches target: recognize and dock with protein

○Tethering → Docking → Fusion

○Specific combination of Rab
proteins and tethering proteins identify membrane type

■Ensure vesicles fuse only
with the correct membrane

○SNAREs: transmembrane proteins
important for docking the vesicle in place

○Vesicle Fusion: deliver its cargo
and adds vesicle membrane to
organelle

●Golgi apparatus: post office of the cell

○Proteins enter on the cis side (closer to the nucleus)

○Proteins get sorted as they travel
through the golgi apparatus

○Proteins leave in vesicles from the
trans side to their designated
destinations