Protein turnover targeting

In chapter 29, Protein Synthesis, Targeting, and Turnover, the processes of protein synthesis and transport are described. First the process whereby amino acids are ordered and polymerized into polypeptide chains is described. Next, posttranslational alterations of newly synthesized polypeptides is considered. This is followed by a discussion of the targeting processes whereby proteins migrate from their site of synthesis to their target sites of function. Finally, proteolytic reactions that result in the return of proteins to their starting materials, the amino acids, are considered. 

Animals are constantly synthesizing and degrading nitrogen-containing molecules such as proteins and nucleic acids. Protein turnover is believed to provide cells with metabolic flexibility, protection from accumulations of abnormal proteins, and the timely destruction of proteins during developmental processes. Ubiquitin is one stress protein that plays an important role in targeting proteins for destruction. 

Photosystem II (PS II) of higher plant possess unique properties with respect to function, organization and protein turnover. It is a multisubunit protein complex which Is composed of at least 20 different polypeptides (1). The two reaction center polypeptides, designated D1 and D2, appear to carry all the redox components necessary for the primary photochemistry of PS II (2) and possibly also the Mn (3). The great majority of the PS II units is located in the appres-sed thylakold regions in association with its chlorophyll a/b antenna (4). PS II has a central catalytic role, but It also plays a central role In the long and short term acclimation of the photosynthetic apparatus. It Is also the target for the photoinhibition process which leads to Impaired electron transport capacity and the subsequent breakdown of the two reaction center subunits. In particular the Dl-protein. 

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