Summary Lecture Notes Cell Biology | Protein Transport | Wageningen | 2025/26

Week 4
Chapter 15 intracellular compartments and protein transport
The cell: a complex, organized unit
 In Eukaryotic cells, different metabolic processes are carried out in
different compartments
 Why compartmentalize?
o Specialization in function
o Higher efficiency
o Portion off processes that are harmful for the rest of the cell
3 ways of protein import by organelles
1. Transport in and out the
nucleus through nuclear pores
 Nuclear proteins are
synthesized by free ribosomes
o Nuclear proteins are fully
synthesized in the cytosol
o After protein folding, the
nuclear proteins are
imported from the cytosol through nuclear pores
 Transport through nuclear pores
o Disordered protein segments fill the center of the channel
allowing small water-soluble molecules to pass in and out freely
and nonselectively
o Prospective nuclear proteins contain a nuclear localization signal
(NLS) consisting of several positively charged amino acids
o Binding of the NLS to a nuclear import receptor guides passage of
nuclear proteins by disrupting the pre meshwork
o Proteins are transported into the nucleus in their fully folded
conformation
o Signal sequence: amino acid sequence that directs a protein to a
specific location in the cell (generally 15-60 amino acids in length
 The positively charged amino acids will locate on one side of
the helix
 The apolar to a other side this combination is recognized by
receptors in the membrane of a mitochondrion
2. Transport across membranes
 Mitochondria and chloroplasts
o Surrounded by two
membranes
o DNA (circular) and ribosomes:
within inner membrane

, o Large percentage of genes transferred to nucleus of host
o Most proteins are encoded by genes in the nucleus and imported
from the cytosol
 Protein import used by mitochondria and chloroplasts
o Most mitochondrial and chloroplasts proteins are imported form
the cytosol
o These proteins are synthesized by free ribosomes in the cytosol
o Proteins typically unfold during this type of transport
o A specific signal sequence (at the N-terminal end) is recognized..
o By an import receptor coupled to a protein translocator
o Proteins unfold during transport
o Proteins are translocated across both membranes simultaneously
o The signal peptide is cleaved off after translocation
 The ER is the most extensive
membrane in eukarytoic cell
functions:
o Attachment site for
ribosomes
o Protein folding and
modification
 The ER: first stop of the
endomembrane system
o Proteins that function in organelles o the ondomembrane system
(ER, Golgi, endosomes, lysosomes and also peroxisomes), or
outside the cell, are formed by ribosomes that are attached to the
rough ER
 Proteins enter the ER while being synthesized
o Two pools of ribosomes
 Free ribosomes
 Membrane-bound ribosomes, bound to the cytosolic side of
the ER lumen
o Ribosomes are directed to the ER membrane by an ER signal
sequence, which binds to a signal recognition particle (SRP) as it
emerges from the ribosome
 Er signal sequence targets proteins to the ER
o SRP and SRP receptor act as molecular matchmakers that guide
ribosomes to the ER
 Signal peptidase frees N-terminus of translocator
 Proteins are released as soluble proteins in the lumen of the ER and
can be transported to other locations in transport vesicles
3. Transport by vesicels
 This type of transport occurs between organelles of the
endomembrane system
 The endomembrane system (ER, Golgi, endosomes, lysosomes and
also peroximes) mainly uses vesicles