Multiprotein signaling complexes

Many signaling proteins are multivalent, with several different binding modules. By combining the substrate specificities of various protein kinases with the specificities of domains that bind phosphorylated Ser, Thr, or Tyr residues, and with phosphatases that can rapidly inactivate a pathway, cells create a large number of multiprotein signaling complexes. 

Houtman, J.C., Barda-Saad, M. and Samelson, L.E. (2005) Examining multiprotein signaling complexes from all angles. FEBS J. 272, 5426-5435. 

Vondriska, T.M., Pass, J.M. and Ping, P. (2004) Scaffold proteins and assembly of multiprotein signaling complexes. J. Mol. Cell Cardiol. 37, 391-397. 

Abstract It is now apparent that multiprotein signalling complexes or signalling machines are... 

Phosphorylation of proteins on Ser/Thr residues is one of the most common regulatory modifications of signaling proteins (see Chapters 2 and 7). Only recently has it been recognized that serine/threonine phosphorylation results in the formation of multiprotein signaling complexes through specific interactions between phosphory-lated sequence motifs and the following phosphoserine/threonine-binding domains (review Yaffe and Elia, 2001). 

Yang, W., Steen, H., Freeman, M.R. (2008) Froteomic approaches to the analysis of multiprotein signaling complexes. Proteomics, 8,832-851. 

Fig. 7.21 Model of a signaling complex organized at the membrane of a target organelle by AKAPs. A schematic representation of a multiprotein complex formed by a prototypic AKAP, protein kinase A and two other effector proteins, protein phosphatase (PPase) and protein kinase C (PKC), both activated by calcium. DAC diacylglycerol.

Protein-protein interaetions are the basis on which the cellular and structure and function are built and interaction partners are an immediate lead into biological function which can be exploited for therapeutic purposes. Proteomics has been shown to make a crucial contribution to the study of protein-protein interactions (Neubauer, Gottschalk et al. 1997). Proteomic approaches to tackle multiprotein complexes usually involve purification of the entire complex by a variety of affinity methods, and protein identification by western blotting or mass spectrometry based approaches. The first proteomic analysis of multiprotein complexes relevant to the brain was the purification and identification of the molecular constituents of the NMDA receptor-adhesion protein signaling complexes (NRC) (Husi and Grant 2001) (Husi, Ward et al. 2000) and this will be described as a prototypic approach to multiprotein complex proteomics. 

During polyadenylation the primary transcript is shortened in an endonucleolytic step and appended with ca. 200 A-residues. The endonucleolytic incision requires two signal sequences on the pre-mRNA. A highly conserved AAUAAA sequence 10-30 nucleotides upstream from the hydrolysis site serves as one signal. Another signal in the form of a less well conserved GU- or U-rich element upstream of the hydrolysis site. Both together constitute the polyadenylation signal (Fig. 1.45). Polyadenylation occurs in a multiprotein complex, whose composition is not yet explained in all details. 

While there is usually only one El enzyme, many species of E2 proteins and multiple families of E3 enzymes or E3 multiprotein complexes exist. Selection of substrates for ubiquitin-ligation occurs mainly by specific E3 enzymes which target substrate proteins that contain specific recognition signals (fig. 2.15B). E3 enzymes also can bind indirectly to the substrate, via an adaptor protein. 

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