Protein sorting I Flashcards
why are post translational modifications necessary
regulate protein biological activity, location and interaction with other molecules
where do PTM occur
at distant amino acid sites
examples of PTM are
proteolysis, glycosylation, phosphorylation, ubiquitination
Proteolysis
is irreversible. Is used for protein sorting and regulating protein activity
glycosylation
oligosaccharide chains (glycan) are added to proteins in the ER and Golgi. it can happen in 2 ways: N-glycosylation or O glycosylation
N-glycosylation
occurs on asparagine residues
O-glycosylation
occurs on serine, threonine residues
what are the functions of glycosylation
correct folding of modified proteins and confer solubility, protect protein from proteolysis, may help cell adhesion, are used as surface signals
phosphorylation
occurs at Ser, Thr, Tyr residues. Is used to switch on/off proteins
ubiquitylation
ubiquitin is a small protein that can be covalently attached to other proteins on Lys residues.
mono ubiquitylation
histone regulation
multi-ubiquitylation
endocytosis
poly-ubiquitylation
proteosomal degradation
where do proteins go after synthesis
stay in the cytoplasm or be sorted to cell compartments
when can protein sorting occur
before or after synthesis
what are necessary for protein sorting
~localization signals: in the protein that must be sorted
~sorting receptor/translocator: guides protein to correct destination
what are the 2 kinds of localization signals
~signal sequence: is a continuous stretch of AA
~ signal patch: separate AA motifs brought together when the protein folds
note that
physical properties of he signal and location within a protein are more important than exact AA sequence
proteins are sorted with 3 main methods
gated, transmembrane, vesicular transport
what kind of transport is used to enter the nucleus
gated transport
how do you enter the nucleus
- through nuclear pores
- small molecules can move by diffusion, macromolecules are transported by active diffusion
- proteins can be transported fully folded
- localization signal is not removed after sorting
the nucleopore is
bidirectional , it imports histones, non histonic proteins, ribosomal proteins.
exports: mRNA and ribosomal units
what molecules are actively transport through NPC
molecules >40kDa, is protein mediated, requires energy
what molecules can freely diffuse through NPC
molecules <40kDa, and diameter<9nm
NPC permeability declines with
age, due to deletion of some nucleoporin, and oxidative damage
what are the 4 main substances involved in Nuclear transport
- proteins of NPC
- nuclear localisation signal, nuclear export signal
- nuclear receptors (karyopherins)
- monomeric Ran GTPase
Nuclear localization signals
are one or two short sequences rich in basic amino acids (Lys Arg)
nuclear export signals
sequences rich in hydrophobic AA - Leu
what are the 2 kinds of nuclear receptors
importins and exportins
nuclear transport requires energy when
when transporting in either direction
where does energy for nuclear pore transport come from
Ran GTPase which has 2 states. Ran GAP (GTP activating protein) and Ran GEF (guanine exchange factor). They make a conformational gradient
explain nuclear import
- importin binds to cargo and NPC and transports it into nucleus
- binding of RanGTP to importin causes cargo release
- importin bound to RanGTP is transported back to cytosol
- hydrolysis of GTP makes RanGDP dissociate from importin
how does nuclear export work
- in the nucleus, exportin binds both RanGTP and cargo and transports it to the cytosol
- hydrolysis of GTP makes Ran GDP dissociate from exportin and cause cargo release
- free exportin goes back to nucleus
