Cell Membrane Transport: Definitions and Key Proteins in Biology

Active transport

Movement of a solute against its gradient using energy, usually through a pump.

Amphipathic

Having both hydrophilic and hydrophobic regions; this is why membrane lipids form bilayers.

Anterograde transport

Forward vesicle traffic through the endomembrane system, classically ER to Golgi to later destinations.

Apoptosis

Programmed cell death; used as the final response if ER stress and quality control fail.

Aquaporin

A membrane channel that allows rapid water movement across membranes.

Asymmetry of membranes

The two sides of a membrane have different lipid and protein compositions and different orientations.

BiP

An ER chaperone that binds exposed hydrophobic regions on misfolded proteins and helps retain them in the ER.

Calnexin

An ER chaperone involved in folding quality control, especially for certain glycoproteins.

Cargo protein

A protein being transported, such as through the NPC or inside a vesicle.

Channel protein

A membrane protein that forms a selective hydrophilic pore for passive transport.

cis-Golgi network

The Golgi side that receives incoming vesicles from the ER.

Cisternae

Flattened membrane sacs that make up structures such as the rough ER and Golgi.

Co-translational translocation

Protein import into the rough ER while the protein is still being translated.

Concentration gradient

A difference in solute concentration across space or across a membrane.

COPI

A coat protein complex mainly used for retrograde transport, especially Golgi back to ER.

COPII

A coat protein complex used mainly for transport from the ER to the Golgi.

Cytosolic face

The side of a membrane that faces the cytosol.

Detoxification

A major peroxisome function involving breakdown of harmful molecules.

Dislocation

Removal of misfolded proteins from the ER into the cytosol for degradation; also called retrotranslocation.

Docking

The early vesicle-target membrane recognition step, typically beginning when Rab-GTP binds a tethering protein.

Electrochemical gradient

The combined effect of a concentration gradient and membrane potential on ion movement.

Endomembrane system

The connected trafficking system that includes ER, Golgi, endosomes, lysosomes, vesicles, and plasma membrane.

ER exit sites (ERES)

Specialized regions of the ER where transport vesicles bud off toward the Golgi.

Facilitated transport

Movement across a membrane with help from a membrane transport protein.

Saturated fatty acid tail

A fatty acid tail with no double bonds; packs more tightly and usually decreases membrane fluidity.

Signal sequence is necessary and sufficient

In the usual targeting logic, the sequence is needed for targeting and can often direct another protein to the same destination.

Tethering protein

A target membrane protein that first catches a Rab-GTP-bearing vesicle during docking.

TIC/TOC complex

The chloroplast import complexes that move proteins across the outer and inner chloroplast membranes.

TIM/TOM complex

The mitochondrial import complexes that move proteins across the outer and inner mitochondrial membranes.

t-SNARE

A target-membrane SNARE that pairs with a v-SNARE to promote membrane fusion.

Translocon

A protein channel in the ER membrane that allows a growing polypeptide to enter the ER or insert into the membrane.

Transmembrane protein

A protein embedded in the membrane with regions exposed on both sides.

Transport protein

A general term for a membrane protein that helps move substances across membranes.

UPR

Unfolded protein response; the ER stress response that increases folding capacity and other corrective measures.

Unsaturated fatty acid tail

A fatty acid tail with one or more double bonds; packs less tightly and usually increases membrane fluidity.

v-SNARE

A vesicle SNARE that pairs with a t-SNARE on the target membrane to drive fusion.

Vesicle

A small membrane-bound transport carrier used to move cargo between compartments.

Sar1

Activated by a GEF; hydrolyzes GTP after budding, which helps the coat come off.

BiP and calnexin

Help retain misfolded or incompletely folded proteins in the ER.

Flippases, floppases, and scramblases

Control lipid distribution between the two leaflets.

Simple diffusion

Works best for small, nonpolar molecules.

Passive transport

Goes down a gradient.

Aquaporins

Greatly speed water transport.

Osmosis

Water moving toward higher solute concentration.