Cell Sorting, Endocytosis, and Exocytosis

Study Notes

Cell Sorting, Endocytosis, and Exocytosis

Cell sorting is essential for proper function, requiring proteins to be in the right place at the right time.
Eukaryotes have multiple subcellular compartments, each with its own specific proteins.
Proteins have specific signal sequences, acting as addresses, to direct them to their correct destinations. Most proteins lack this signal, which means they will remain in the cytosol.
Amino acid sequences (sorting signals) may be continuous or discontinuous and can be removed after sorting.
Sorting signals are recognized by complementary receptor proteins.
Proteins with nuclear import sequences will be directed to the nucleus.
The size cutoff for transporting molecules is below ~60kDa which will diffuse across the nuclear membrane; active transport is needed for larger molecules including DNA/RNA polymerases & ribosomal subunits (above ~60kDa) .
Transmembrane transport moves proteins across membranes using membrane-bound translocators; the proteins are unfolded to snake through the translocators.
Proteins destined for secretion or to be inside other organelles are fully transported across the membrane (soluble proteins).
The signal peptidase removes the signal sequence and releases the fully transported proteins into the ER lumen.
Transmembrane proteins are partially moved across the membrane, remaining embedded in the membrane.
Co-translational transport moves proteins into the ER; signal recognition particles (SRPs) bind to signal sequences, attracting ribosomes to the ER membrane, and binding to the translocator, causing the SRP to release. Polypeptides then translocate through the membrane.
Soluble proteins have their signal peptide cleaved off by signal peptidase after transport into the ER lumen.
Transmembrane proteins have signal sequences and stop-transfer sequences, which cause hydrophobic amino acids to form transmembrane domains.
Vesicular transport uses 'membrane bags' (vesicles), relies on budding and fusion, and moves proteins to compartments like the ER, Golgi, and cell surface.
Proteins on the lumenal side of vesicles remain lumenal.
Vesicular transport is crucial for protein export.
Vesicle transport relies on coat proteins, Rab GTPases, and SNAREs.
Endocytosis is the process of cells taking in material from outside the cell, using vesicles.

Endocytosis

Endocytosis relies on vesicles.
Two main types include pinocytosis (drinking) and phagocytosis (eating).
Pinocytosis involves vesicles less than 150 nm taking up dissolved substances.
Receptor-mediated endocytosis involves specific receptors binding to molecules.
Phagocytosis involves vesicles greater than 250 nm taking up large particles, for example, bacteria.
Specialised cells engulf large particles using phagocytosis

Endocytosed Cargo Pathway

Endocytosed cargo generally passes through the pathway PM→Early Endosome→Late Endosome→Recycling Endosome→PM pathway.
Early endosomes (EE) sort cargo, and late endosomes/multivesicular bodies (MVB) prepare cargo for degradation or secretion.
Lysosomes degrade cargo.
Recycling endosomes return cargo to the plasma membrane (PM).

Phagocytosis

Specialized phagocytic cells engulf large particles (e.g., bacteria) via receptor-mediated endocytosis.
Phagocytic cells have pseudopods that extend around particles, engulf them into a phagosome.
Lysosomes fuse with phagosomes, degrading particles.
Phagocytosis also plays a role in removing dead cells.

Lysosomes

Lysosomes contain acid hydrolases, enzymes that digest endocytosed material at an acidic pH.
Proteins and other large, digested particles from phagocytosis or endocytosis are processed and broken down in the lysosomal compartment, which contains acid hydrolases.

Secretory Pathway Overview

The secretory pathway involves transferring proteins and lipids from the endoplasmic reticulum (ER) to the Golgi apparatus and then to the plasma membrane.
Proteins enter the ER cotranslationally or posttranslationally, move through the Golgi, and are then secreted via exocytosis.

Summary

Protein transport pathways are facilitated by targeting signals that are part of the amino acid sequence of the protein.
Protein folding quality control and modifications are essential for function, in the ER or the Golgi apparatus.
Proteins and lipids move through the intracellular compartments using vesicular transport and via proteins like coat proteins, Rab GTPases, and SNAREs.

Changes to Protein Structure During Secretion

Protein modifications, such as N-linked and O-linked glycosylation, enhance protein stability and function.
ER chaperones also aid in quality control of proteins.

Key Proteins in Vesicular Trafficking

Vesicular trafficking involves coat proteins, Rab GTPases, and SNAREs.

Description

This quiz covers the processes of cell sorting, endocytosis, and exocytosis, focusing on the mechanisms and proteins involved. Learn about the importance of sorting signals and how proteins are directed to their proper cellular locations. Understand the differences between passive and active transport for various molecules within eukaryotic cells.