Activation segment

Figure 13.30 Ribbon diagram of the structure of Src tyrosine kinase. The structure is divided in three units starting from the N-terminus an SH3 domain (green), an SH2 domain (blue), and a tyrosine kinase (orange) that is divided into two domains and has the same fold as the cyclin dependent kinase described in Chapter 6 (see Figure 6.16a). The linker region (red) between SH2 and the kinase is bound to SH3 in a polyproline helical conformation. A tyrosine residue in the carboxy tail of the kinase is phosphorylated and bound to SH2 in its phosphotyrosine-binding site. A disordered part of the activation segment in the kinase is dashed. (Adapted from W. Xu et al.. Nature 385 595-602, 1997.)...

Figure 13.32 Regulation of the catalytic activity of members of the Src family of tyrosine kinases, (a) The inactive form based on structure determinations. Helix aC is in a position and orientation where the catalytically important Glu residue is facing away from the active site. The activation segment has a conformation that through steric contacts blocks the catalytically competent positioning of helix aC. (b) A hypothetical active conformation based on comparisons with the active forms of other similar protein kinases. The linker region is released from SH3, and the activation segment changes its structure to allow helix aC to move and bring the Glu residue into the active site in contact with an important Lys residue.

In the activation segment an Arg binds to phosphotyrosine, another Arg binds to the 7 phosphate of ATP and an Asp binds to the Mg atom in the active site. (Adapted from F. Sicheri et al.. Nature 385 602-609, 1997.)... 

An incident has just been reported. How is a team activated The team activation segment of a company s incident investigation management system should guide the user to assemble a team based upon the following two areas of concern ... 

Simultaneous IPN. According to the statistical theory of rubber elasticity, the elasticity modulus (Eg), a measure of the material rigidity, is proportional to the concentration of elastically active segments (Vg) in the network [3,4]. For negligible perturbation of the strand length at rest due to crosslinking (a reasonable assumption for the case of a simultaneous IPN), the modulus is given by ... 

Actually, crosslinks control the molecular packing and indeed significantly affect the elastic modulus of the material. As the intermolecular energy of kink formation is also determined by elastic modulus, the yield stress will definitely vary with modulus and thus the cross -linking density. In other words, crosslinks may not seriously affect the activation segment configuration in the molecular chain but will indirectly control the yield stress. 

Many protein kinases require phosphorylation of Ser/Thr or Tyr residues to reach full activity. The activating phosphorylation often takes place in a part of the structure in the vicinity of the active center, known as the activation segment which spans two conserved sequence motifs (DFG to APE, using single-letter code) present im almost all kinases (review Johnson et al., 1996). Phosphorylation of the activation segment may be catalyzed by other protein kinases or by the protein kinase s own active center. In the latter case, this is generally an autophosphorylation in trans, i.e., between the subunits of an oligomeric protein kinase (see Chapter 8.1.3). 

Ser/Thr (e.g. Thrl97 of PKA) or Tyr phosphorylation sites (see insulin receptor. Chapter 8) are located at the activation segment. As shown for the phosphorylation of the CDK2-cyclin A complex (see 13.2.4), phosphorylation in the activation segment leads to reorganization of the catalytic center in the sense of an optimal orientation of the catalytic groupings (review Johnson and O Reilly, 1996). 

It is assumed that the catalytic domain of the receptor can exist in two conformations. In the inactive conformation, the activation segment lies in the catalytic center, whereas in the active conformation, it swings out of the active center and both the ATP binding site and the binding site for protein substrate are free and accessible. According to this model, the equilibirum of the two conformations lies on the side of the inactive form in the non-phosphorylated state phosphorylation at Tyrll62 shifts it towards the active form. 

Phosphorylation of the CycA-CDC2 complex at ThrlhO leads to a near 300-fold increase in protein kinase activity. Thrl60 of CDK2 hes in the activation segment (also known as the T loop) that blocks the access to the substrate binding site in the inactive... 

Nolen, B., Taylor, S., Ghosh, G. Regulation of protein kinases controlling activity through activation segment conformation. Mol. Cell 2004, 15, 661-675. 

The term cosd" becomes relatively small due to the significant increase in a at temperatures where active segmental motions occur. If the value of the product... 

Molecular basis for the dephosphorylation of the activation segment of the insulin receptor by protein tyrosine phosphatase IB. Mol Cell 6 1401-1412... 

Overall, the protein kinase structures contain several flexible elements that can be fixed in either an active or an inactive conformation. The flexible hinge between the two lobes allows for their regulated movement. Other highly mobile elements are the activation segment and, to a lesser extent, the C-helix. The structural information on the active state of several protein kinases shows a very similar structure of the catalytic domain. The following structural features are characteristic of the activated state of protein kinases ... 

The phosphorylated activation segment helps to organize the catalytic residues for optimal phosphate transfer. 

The two other multifunctional CaM kinase of types I and IV are, apart from Ca2+/ calmodulin control, regulated via phosphorylation by an upstream CaM kinase kinase, CaMKK (review Soderling, 1999). The CaMKKphosphorylates CaM kinase I and IV in the activation segment and thereby greatly enhances the activity of these enzymes. Examples of substrates of CaM kinases I and IV are transcription factors, the MAP kinases and adenylyl cyclase. 

Enhancement of the catalytic activity is achieved by autophosphorylation in the activation segment within the kinase domain. It is generally assumed that this autophosphorylation takes place by a trans mechanism. Accordingly, two neighboring Tyr kinase domains in the receptor oligomer perform a mutual phosphorylation (see Fig. 8.3). 

A total of seven tyrosine residues become phosphorylated during autophosphorylation. Two of these are located in the vicinity of the transmembrane element, three are in the activation segment of the catalytic domain, and a further two in the region of the C terminus. 

From the temperature factors derived from the crystallographic analysis, it can be concluded that the activation segment is quite mobile and can exist in different conformations in solution. The equilibrium between the multiple conformations appears to be shifted to a catalytically active conformation in the phosphorylated state that is fixed by interactions with other structural parts of the catalytic center. 

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