BIO 457 Week 8 Vocab

Endoplasmic Reticulum (ER)

Most extensive compartment of the endomembrane system, subdivided into the Rough ER, Smooth ER, or other sub-compartments like the Sarcoplasmic Reticulum

Rough ER

Rough b/c of the association of ribosomes engaged in translation of proteins destined for secretion or other endomembrane fates

Smooth ER

Where much of lipid biosynthesis takes place, among other functions

Sarcoplasmic Reticulum

Sub-compartment of ER located in skeletal muscle

Golgi Apparatus

Modifies and packages proteins for transport to other regions of the cell. The function and mechanisms are still not entirely understood.

Order of compartments

1. Cis-Golgi Network

2. Cis-

3. Medial

4. Trans-Golgi Cisternae

5. Trans-Golgi Network

Secretory Vesicle

Proteins destined for secretion outside of the cell by Exocytosis are packed in vesicles w/ a characteristic size and equipment. Some follow the Constitutive Secretory Pathway, meaning they are exocytosed more or less as soon as they meet the membrane and wait for a specific trigger to fuse.

Endosomes

Membrane vesicles created by Endocytosis join a subset of Golgi-derived vesicles to create endosomes, which are small and irregular membrane bound sacks that have multiple functions:

1. Recycle

2. Regulate signal transduction

3. Direct things back to the Golgi

4. Fuse w/ lysosomes

Lysosomes

Last stop on the secretory line, acidified membrane compartments in which acid-activated enzymes chew up proteins, lipids, and just about everything else that gets into them. For cells that live by eating, they are the digestive apparatus of the cell.

They must include the means to transport digestion products out to the cytoplasm

Terminal mediators of Autophagy

In many eukaryotes (other than animal cells), a greatly-expanded one is called a Vacuole, and acts as a hydraulic power reservoir

Phagosomes

Not endosomes; don’t form from a vesicle-coating process, but rather from the cell membrane extending in an actin-dependent manner to enwrap some large object for ingestion, process we call Phagocytosis

Fuse w/ lysosomes to initiate digestion

Vesicle

Membrane-bound, fluid-filled bubble that is budded from another larger membrane compartment

Formation requires coating proteins

Coat Proteins

Include Clathrin, COP1, COPII, and retromer. These are recruited to membrane patches by Coat-recruitment GTPases. These include small GTPases Sar1 and Arf, which are activated by compartment specific GEFs in response to sufficient available cargo.

The assembling coat deforms the membrane into a bud, but to pinch it off into a separate vesicle requires dynamin

Dynamin

A large GTPase that assembles a multimeric coil around membrane tubules. GTP hydrolysis changes the pitch and diameter of the coil to pinch the tubule to the point that membrane scission is favored. Incidentally, the modern eukaryote uses it to divide mitochondria too

SNAREs

Compartment-specific pairs of vesicle fusion proteins, v-SNAREs and t-SNAREs (v is for vesicle, t is for target) recognize each other, entwine, and winch vesicles down to target membranes tightly enough that vesicle fusion is likely

Rabs

Largest family of small GTPases, act as address labels. There are dozens of them.

Activated (GTP-bound) Rabs are loaded onto transport vesicles along w/ SNAREs, and of course the coat recruitment GTPases. The Rab is target and compartment-specific.

Compartments have specific Rab GEFs. This means that targets have to have specific Rab effectors which act as tethering proteins, docking proteins even Rab GAPs. Some Rab effectors recognize vesicles and attach them to motors for transport.

Signal Recognition Particle (SRP)

An ancient part of physiology that we share w/ prokaryotes

Detects the signal sequence at the N-terminus of newly-translating proteins as they emerge from the ribosome. Binding of (TERM) to the translating ribosome pauses translation until the (TERM) and ribosome dock to an aqueous pore on the ER, and then stuff the nascent protein chain into that pore. Further translation extrudes the protein into the ER Lumen.