Lec 14

A newly incorporated membrane protein in the ER has what possible fates?

It may be a resident in ER membrane, or be transported to another endomembrane system

Endomembrane system

A series of membrane-bound organelles in eukaryotic cells, including the ER, Golgi apparatus, lysosomes, and vesicles, that work together to modify, package, and transport lipids and proteins.

Golgi complex

Processes, sorts, and packages proteins and lipids received from the ER.

Acidification of the organelle lumen

H+ is pumped into the lumen to create an acidic environment essential for enzyme activity and proper functioning of organelles.

pHluorin

GFP variant that exhibits pH sensitive fluorescence and
can be used to measure pH in the organelles of live cells

Compartments of the Golgi complex

Consists of the cis-Golgi network (CGN), medial cisternae, and trans-Golgi network (TGN), each with specific functions.

Where does most protein processing occur

Medial cisternae of golgi

Forward biosynthetic/processing route

ER —>cis-Golgi —>medial-Golgi —>trans-Golgi

Cisternal maturation model

Each golgi cisterna doesn’t keep its own identify and matures overtime

Stationary Cisternae model

Golgi as a stable, stationary structure.
Each Golgi cisterna has its own identity

How does the ER keep it’s identity through the continuous movement of its content?

It has specific resident proteins with retrieval tags

Proteins whose final destination is in the ER has a ____

KDEL sequence

KDEL sequence

Once golgi has processed protein, it will use this tag to deliver it back into the ER

Retrograde flow of the ER

involves the movement of proteins and lipids from the Golgi apparatus back to the endoplasmic reticulum, ensuring the retention of ER components.

Once processing occurs in the golgi, what are the options for proteins

Lysosomal sorting
Secretion to plasma membrane
Secretory vesicles

Lysosomes

The principal sites of intracellular digestion.

Contents of lysosomes

Contain various hydrolases that are active at low pH (acid hydrolases).

Function of lysosomes

Degrade unwanted proteins and organelles through processes like autophagy.

Role of endosomes

Involved in sorting and transporting proteins from Golgi to lysosomes.

Lysosomal targeting signal

Involves addition of mannose 6-phosphate (M6P) groups to lysosomal enzyme precursors in the Golgi network.

Transportation of M6P-tagged hydrolases

Recognized by M6P receptors for transport to endosomes and lysosomes.

Autophagy

A cellular process that degrades and recycles damaged organelles and proteins to maintain homeostasis.

Exocytosis

The process by which intracellular vesicles fuse with the plasma membrane to release their contents outside the cell.

Regulated exocytosis (Secretory pathway)

is a mechanism where secretion of molecules is controlled by specific signals, often in response to hormones or neurotransmitters.

Unregulated exocytosis (Constitutive secretory pathway)

is a continuous process where cells secrete substances regardless of external signals, allowing for constant release of materials.

Example of regulated exocytosis

GLUT4 translocation to plasma membrane upon insulin signaling.

GLUT4

Glucose uniporter, (into cell)

What happens when insulin binds t the insulin receptor

GLUT4 is translocated to membrane by endosome to increase glucose intake

Another example of regulated exocytosis

Synaptic vesicle exocytosis upon neuronal depolarization.

Synaptic Vesical exocytosis

Ca2+ enters the cell causes release of neurotransmitters into the synaptic cleft.

Endocytosis

The process through which cells internalize substances by engulfing them in a membrane-bound vesicle.

Endocytic pathways

Include pinocytosis, macropinocytosis, receptor-mediated endocytosis, and phagocytosis.

Pinocytosis

Uptake of plasma membrane along with extracellular fluid (cellular drinking).

Macropinocytosis

Uptake of extracellular fluid on a large scale (non-specific).

Receptor-mediated endocytosis

Import of select extracellular macromolecules via specific receptors.

Phagocytosis

Ingestion of large particles (cellular eating), such as engulfment of red blood cells by macrophages.