Download Key concepts_Protein processing and modification

Key concepts Protein processing and modification
Even after translation, many proteins require further processing to be directed to appropriate
organelles or to fulfill their physiological function. Such processing may involve covalent
cleavage and/ or splicing of the chain, or modification of residue side chains.
Much processing and modification occurs in the endoplasmic reticulum or the Golgi apparatus;
this requires directed transport across membranes.
Transport of proteins across membranes, known as translocation, utilizes multiprotein
complexes called translocons. A number of different mechanisms are employed in bacteria and
eukaryotes. In particular, proteins can be translocated either directly from the ribosome, in cotranslational translocation, or from the cytoplasm, in post-translational translocation.
The vast majority of protein processing involves specific modifications of amino acid side
chains. These reactions are catalyzed by specific enzymes and are often reversible.
Phosphorylation and dephosphorylation are widely employed in enzyme regulation and in cell
signaling. In some cases, there are cascades of successive enzyme activations.
Phosphorylation occurs primarily at serine and threonine residues. Acetylation of lysine residues
is mostly used to modulate interactions between proteins or between proteins and nucleic acids.
Glycosylation involves the addition of saccharide or polysaccharide moieties to serine or
threonine, in O-linked glycosylation, or to asparagine, in N-linked glycosylation. Such
modification serves a wide variety of functions, including recognition, as in blood group factors.
Ubiquitylation is the covalent addition of a small protein, ubiquitin, to lysine side chains of
proteins. Monomers or linear or branched polymers of ubiquitin can be added. Ubiquitylation
often serves as a marker for protein degradation by the proteasome. Proteins themselves carry
information that determines the addition of the ubiquitin degradation signal.