Complexes protein-substrate

Protein tyrosine kinases (PTKs) are enzymes (EC 2.7.1.112) that catalyze the transfer of the y-phosphate group of ATP to tyrosine residues of protein substrates. The activity of PTKs is controlled in a complex manner by posttranslational modifications and by inter- and intramolecular complex formations. 

Figure 11.2 Structure of the insulin receptor (a). Binding of insulin promotes autophosphorylation of the (3-subunits, where each (3-subunit phosphorylates the other (3-subunit. Phosphate groups are attached to three specific tyrosine residues (tyrosines 1158, 1162 and 1163), as indicated in (b). Activation of the (3-subunit s tyrosine kinase activity in turn results in the phosphorylation of various intracellular (protein) substrates which trigger the mitogen-activated protein kinase and/or the phosphoinositide (PI-3) kinase pathway responsible for inducing insulin s mitogenic and metabolic effects. The underlying molecular events occurring in these pathways are complex (e.g. refer to Combettes-Souverain, M. and Issad, T. 1998. Molecular basis of insulin action. Diabetes and Metabolism, 24, 477-489)...

Elucidation of the structure of the SCF ubiquitin E3 ligase complex and their substrates should help resolve certain issues regarding the mechanism by which SCF E3 ligase promotes ubiquitin transfer from the E2 enzyme to the protein substrate. Recently, the structures of Cull/Rocl in complex with Skpl and Skp2, as well as that of Skpl/jS-Trcp and Cdc4, have been reported [21, 25, 55, 56]. These studies... 

Eukaryotic genomes contain information for more than 20 E2s and hundreds of E3s. In contrast to the wealth of components devoted to marking protein substrates for destruction, only one enzyme, the 26S proteasome, has been found to degrade ubiquitylated proteins. However, there is complexity here as well, since the 26S proteasome is an assemblage of at least 30 different subunits. Moreover, there is a growing list of proteins that act as proteasome activators, adapters, or accessory factors. In this chapter I focus on basic biochemical and physiological properties... 

The term proteasome is used to describe two kinds of multisubunit proteolytic complexes, the 26S and 20S, based on their sedimentation coefficient. The 26S proteasome degrades ubiquitinated protein substrates. The 26S complex contains the 20S as a core and regulatory caps on either end like a dumb bell. Each cap of the 26S proteasome is known as the 19S regulatory complex (19S RC). The 20S core is a cylindrical structure consisting of the catalytic part of the proteasome. ... 

Panlp- Panlp in yeast, and its mammalian homologue, EGFR protein substrate 15 (EpslS), are essential for normal endocytosis (Carbone et al. 1997 Benmerah et al. 1998 Wendland and Emr 1998). Although these proteins are associated with clathrin complexes and genetic evidence raised the possibility that Panlp may act as an adaptor connecting RspSp to potential ubiquitination substrates, physical associations between RspSp and Panlp have not been detected. In mammalian cells, EpslS is tyrosine-phosphorylated and mono-ubiquitinated upon EGF stimulation (van Delft et al. 1997). The tyrosine-phosphorylation of the protein may play a role in... 

Some ubiquitin ligase activities are coupled with large-multisubunit protein complexes. The substrates of these E3 complexes represent a broad spectrum of proteins that participate in a variety of cellular functions, e.g. regulation of CDK activity, activation of transcription, signal transduction, assembly of kinetochores, and DNA replication. Concerning those substrates relevant for the cell cycle the critical issue is how and when these proteins are ubiquitinated. Studies of cell cycle regulation have demon-... 

With the help of NMR measmement, it has been shown that the Ca Vcalmodulin complex has a flexible structure. Flexibility is probably of great importance for the function of Ca Vcalmodulin. In the complex with the protein substrate (Fig. 6.10b), Ca V calmodulin has a collapsed structme in which the two globular domains are much closer together than in free Ca Vcalmodulin. 

More complex, second order interactions may be imagined, involving more than one natural enemy. For example, consider insects to which tannins are important deterrents and digestion inhibitors. As mentioned above, elevated gut pH appears to be a way of dealing with tannins, since tannin-protein complexes are dissociated or inhibited at alkaline pH (16,32). Indeed, using a model in vitro system in which hemoglobin is employed as a protein substrate, we found that several natural tannins and phenolic extracts do not precipitate this protein when the pH exceeds about... 

---