Posttranslational events

DNA sequencing reveals the order in which amino acids are added to the nascent polypeptide chain as it is synthesized on the ribosomes. However, it provides no information about posttranslational modifications such as proteolytic processing, methylation, glycosylation, phosphorylation, hydroxylation of prohne and lysine, and disulfide bond formation that accompany mamra-tion. While Edman sequencing can detect the presence of most posttranslational events, technical hmitations often prevent identification of a specific modification. 

The biosynthesis of collagen is complex, featuring many posttranslational events, including hydroxyla-tion of prohne and lysine. 

The interplay between posttranslational modifications and the ligand potentially leads to a myriad of functional outcomes for the nuclear receptors. We are only begiiming to map out the relationships between individual posttranslational events and to understand the specific effects of their combinations on receptor activity. Numerous studies highlight the importance of the histone code or how posttranslational modifications of histone proteins affect transcriptional state of the chromatin and dictate transcriptional competency of genes. The abundance of posttranslational modifications on nuclear receptors suggests a similar idea of regulation. 

NRPS biosynthesis differs substantially from ribosomally encoded peptides. In the NRPS biosynthetic strategy, the substrates and intermediates are covalently linked to an active site serine on the thiolation (T) domain of an NRPS via the phosphopantetheinyl arm.23 This posttranslational event is accomplished by a phosphopantetheinyl transferase (PPTase), which primes the active site serine with a portion of coenzyme A (CoA) to generate the holo form of the T domain (Figure 2(b)).24 In traditional NRPS biosynthesis, amino... 

Precursor Processing. Hie precursor protein is synthesized on stromal lamellae and is processed there to the 32kDa mature form (21). Processing of the precursor is a posttranslational event (12) and takes place at the carboxy terminus (22). Following processing, the mature protein translocates to spatially-distinct chloroplast membranes, the grana, where functional photosystem II reaction centers are mainly located (21, 23). 

Thus, the differential expression of genes is required for the production of different proteins because each protein controls a distinct function. The function of many proteins is listed in Table 1.1. In addition, the protein profile of a cell can vary depending on the different kinds of modification of the same protein such modifications of protein may involve acetylation, phosphorylation, glycosylation, or association with lipid or carbohydrate molecules. These modifications in proteins occur as posttranslational events and alter the function of proteins. One example is the mitosis activator protein (MAP) kinase protein controlling the mitosis this protein is activated by phosphorylation to give MAP Kinase (MAPK), MAP kinase kinase (MAPKK), and MAP kinase kinase kinase (MAPKKK). The role of protein modification in the control of cellular activity is discussed later in this book. 

Among the replicative differences between eukaryotic cells (of the viral host) and the pathogenic virions is that the latter need virus-dependent proteinases to produce the mature infectious virus. That is, HIV protease is an essential enzyme utilized by the virus to process proteins in order to reproduce itself. Retroviruses (and RNA viruses) produce the structural and enzymic proteins as large multidomain polyproteins. The specific cleavage of these various domains into the final products is a posttranslational event. These end-products enable the virion to develop into the actual infectious agent (Kransslich and Wimmer, 1988). 

Sulfation of amino acids is a ubiquitous posttranslational modification of peptides and proteins [200]. Interest in the functional importance of this posttranslational event is relatively new and therefore synthetic methods for obtaining sulfated peptides have been developed only recently and have mainly focused on the preparation of sulfated tyrosine-containing peptides. 

De Vitry C, Olive J, Diapier D et al. Posttranslational events leading to the assembly of photosystem II protein complex - a smdy using photosynthesis mutants from Chlamydomonas reinhardtii. ] Cell Biol 1989 109 991-16. 

This retarded folding property is uniquely attributed to the presence of the leader sequence and is deemed to be an essential and perhaps general feature to secure a transport competent conformation. The translocation of the precursor /3-lactamase across the (inner) membrane is a posttranslational event (as opposed to cotranslational process) and does not require the presence of the C terminus or the processing of the signal peptide (61). 

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