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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.