Slide 7.3

Progression and Maturation of Organelles

  • Functionality Sequence

    • Start with RAB (Ras-like GTPase) functions: RAB A, RAB B, RAB C, etc.

    • Unidirectional progression.

    • The maturing organelle's RAB proteins change as it matures.

Vesicular Transport Mechanism

  • Vesicles and SNARE Proteins

    • V-SNAREs (vesicle-soluble N-ethylmaleimide-sensitive factor attachment protein receptors) are found on vesicles.

    • T-SNAREs are found on target compartments.

    • V-SNARE and T-SNARE interact to form a trans-SNARE complex.

    • Energy does not need to be consumed in this merging process.

    • The formation of the trans-SNARE complex lower energy compared to the individual SNAREs.

  • Function of Trans-SNARE Complex

    • Brings membranes closer together.

    • Releases energy that aids in membrane fusion.

Transport Mechanisms and Organelle Maturation

Transition Between ER, Golgi, and Lysosomes

  • Formation of Transport Vesicles

    • Tubular and vesicular clusters are created by the fusion of uncoated and coated vesicles from the ER.

    • Vesicles undergo homotypic fusion (two like structures fusing).

    • These clusters function as transport containers from the ER to the Golgi apparatus.

  • Retrograde Transport

    • Retrains the top one from vesicular clusters (coated vesicles) for maturation towards the cis-Golgi.

    • POP1 is responsible for returning resident ER proteins, such as receptors and transmembrane proteins.

    • POP1 recognizes and binds specific motifs on membrane-bound ER resident proteins.

    • Recruitment of POP1 differs from POP2 and clathrin, as it assembles as a complex, combining inner and outer coats.

    • KdL receptor's cargo affinity correlates to pH levels; higher in acidic environments creating stronger binding.

Structure of Golgi Apparatus

  • Components of Golgi

    • Comprised of cis (entry phase) and trans (exit phase) networks, and medial compartments.

    • Cis-Golgi network (CGN), medial stack (M), and trans-Golgi network (TGN).

  • Primary Functions

    • Modifying oligosaccharides.

    • Sorting proteins and lipids for transport.

Glycoprotein Modification in Golgi

  • Oligosaccharide Processing

    • Complex oligosaccharides are trimmed to core carbohydrates.

    • Enzymes modify oligosaccharides, adding sugars as needed.

    • N-acetylglucosamine (GlcNAc) transferase helps add enzymes as oligosaccharide moves through the Golgi.

  • Distinction Between Complex and High Mannose Oligosaccharides

    • High mannose is endo H sensitive; complex oligosaccharides are resistant.

    • Utilized scientifically to determine oligosaccharides’ location in the processing pathway.

Models of Golgi Maturation

  • Cisternal Maturation Model vs. Vesicle Transport Model

    • Cisternal Maturation Model

    • Entire compartments mature, which contain the cargo.

    • Retrograde transport assists in receptor recycling.

    • Vesicle Transport Model

    • Cargo is matured through vesicle transport between different compartments.

    • Both models utilize acid hydrolyetic enzymes that are optimally functioning in acidic conditions.

Maturation of Acid Hydrolytic Enzymes

  • Getting Enzymes to Lysosomes

    • Transport initiated by precursor proteins co-translationally inserted into the ER.

    • Oligosaccharides are transferred to these proteins by oligosaccharide transferase.

    • During their journey, the oligosaccharides transit to the high mannose form in the Golgi.

    • Phosphotransferase recognizes a specific signal on mature proteins, adding mannose-6-phosphate (M6P).

    • Uncovering enzyme afterward removes the terminal sugar to finalize M6P addition.

    • M6P receptors recognize and facilitate transport to late endosome/lysosome.

Endosomal Pathways

  • Receptor-Mediated Endocytosis, Phagocytosis, and Autophagy

  • Four Endocytic Pathways

    1. Receptor-mediated endocytosis

    2. Pinocytosis

    3. Phagocytosis

    4. Autophagy

Receptor-Mediated Endocytosis

  • Mechanism

    • AP2 protein binds phosphatidylinositol (PIP) to coat vesicles during dephosphorylation.

    • Vesicles transport to early endosome facilitated via RAB proteins and snares.

    • Maturing endosomes transition to late endosomes, eventually fusing with lysosomes.

Pinocytosis

  • Description

    • Non-selective uptake of fluids and solutes through membrane invagination.

    • Materials are thereafter coordinated with lysosomes for digestion purposes.

Phagocytosis

  • Specific Form of Endocytosis

    • Ingest large materials (e.g., microorganisms).

    • Cells forming these structures are termed phagocytes (e.g., macrophages, neutrophils).

    • Cargo-triggered, meaning that the presence of target materials causes membrane rearrangement leading to capture.

Autophagy

  • Purpose and Mechanism

    • Involves engulfing large cellular components for degradation when conditions (e.g., nutrient deprivation) arise.

    • Specialized vesicles (autophagosomes) form around cellular debris.

    • Fusion occurs with lysosomes for degradation.

  • Regulated and Selective Autophagy

    • Removal of specific cellular structures (e.g., damaged organelles) through mitophagy.

Conclusion and Summary of Endocytic Pathways

  • Endocytic pathways (receptor-mediated endocytosis, pinocytosis, phagocytosis) bring extracellular materials into cells for lysosomal degradation.

  • Systems vary by the material size and mechanism employed:

    • Receptor-mediated is highly selective;

    • Pinocytosis is nonselective uptake;

    • Phagocytosis is for larger particles.

  • Regulated Pathways

    • Specialized cells can manage secretion and degradation processes better than standard cells.