Domain families signaling pathways

The four mammalian ARs are members of the rhodopsin-like Class A family of GPCRs, which contain seven transmembrane helical domains ( TMs). Character istics of the four subtypes of the human ARs, length of their primary sequences, their chromosomal localization, and their signaling pathways are given in Table 1. The A2a receptor is considerably longer than the other three subtypes, due to its extended carboxy-terminal. 

The proteins of the other family are also known as BH3 proteins, since they only contain the BH3 domain. An important member of the BH3 family is the Bad protein which is part of the PI3-kinase/Akt signaling pathway (see 15.6). 

The translation of the peptide sequence to functions in a protein molecule can be expressed as modular units (motifs and domains) that confer similar properties or functions in a variety of proteins. Based on characteristics of modular units, proteomes can be classified into families of proteins possessing related functions. For instance, some proteins play a key role in intercellular signaling pathways and some are structural, while others participate in metabolism. Up to 40% of proteins are encoded by the genome and carry out unknown functions, which still need to be discovered. 

There are two distinct types of TNF receptors, TNF-R1 (CD120a or P55) and TNF-R2 (CD120b or P75). They are implicated in inflammatory processes and both belong to the TNF receptor superfamily. TNF receptors are transmembrane proteins with intracellular domains that lack intrinsic enzymatic activity, and consequently, they require cytoplasmic proteins that help initiate the receptor-induced signaling pathways. TNF-R1 possesses an intracellular death domain and TNF-R2 interacts with molecules of the TNF receptor-associated factor 2 family (TRAF). 

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