Posttranslational modifications phosphorylation labeling

Amino acid sequence analysis of the human 5-HT1B receptor has revealed consensus phosphorylation sites in all intracellular loops for PKA and PKC. Phosphorylation of the 5-HT1B receptor was demonstrated by metabolic labeling of the 5-HTib receptor-expressing Sf9 cells with [32Pi]phosphate (141). This posttranslational modification was proposed to be involved in receptor regulation such as desensitization. 

Since these early observation on the in vivo modification of stress-70 proteins, there has been further work documenting phosphorylation of several members of the stress-70 family, both in vitro and in vivo, and further exploration of the labeling of BiP by [ Hjadenosine in vivo. However, it is not yet clear what the significance of posttranslational modification of stress-70 proteins may be for regulating their activities. 

The labeling of BiP in mouse myeloma B cells by [ H]adenosine, suggestive of ADP-ribosylation, has been shown to be a modification found on BiP free of immunoglobulin heavy chains, but not on BiP bound to heavy chains (Hendershot et al., 1988). Additionally, the [ H]adenosine labeling parallels BiP phosphorylation in vivo conditions that increase BiP production decrease BiP phosphorylation and [ HJadenosine incorporation, suggesting that these two posttranslational modifications might modulate BiP activity in concert in some manner. 

It is of special interest to label serine and threonine residues, due to their role as acceptors for posttranslational modifications, namely, for phosphorylation. 

The step labeled p in Figure 1 represents modification of primary proteins to render them functional examples would be posttranslational covalent modifications (e.g., phosphorylation) and binding with other proteins or other molecules. Represented within the set of steps p are the many regulatory events (other than transcription and translation) affecting gene expression and the overall physiology of the cell. 

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