Recognition signal

Figure 8.15 Sequence-specific protein-DNA interactions provide a general recognition signal for operator regions in 434 bacteriophage, (a) In this complex between 434 repressor fragment and a synthetic DNA there are two glutamine residues (28 and 29) at the beginning of the recognition helix in the helix-turn-helix motif that provide such interactions with the first three base pairs of the operator region.

Flynn, f. M., Neher, S. B., Kim, Y. I., Sauer, R. T., and Baker, T. A. Proteomic discovery of cellular substrates of the ClpXP protease reveals five classes of ClpX-recognition signals. Mol Cell 2003, 11, 671-83. 

Many proteins are formed as inactive precursors and become activated by proteolysis. The inactive precursors are termed proenzymes, zymogens or - for hormones like e.g. insulin - prehormones. Processing to the active form occius in a cell- and tissue-specific way and usually requires a specific protease. Activation can also occur intramolecu-larly by autoproteolysis. In most cases, short sequences of the protease substrate serve as a recognition signal for the attack of the processing protease. Of the numerous examples of proteolytic processing of proteases only the digestive proteases will be discussed in more detail. 

Two of the four different E3 enzymes mentioned in the text are shown. The cyclosome/anaphase promoting complex (APC) ligates ubiquitin to regulatory and structural cell cycle proteins containing a destruction box as recognition signal (see also 13.3.2). The activity of the APC is thought to be controlled by phosphorylation. Eor simplicity, the subunits of APC are not shown. 

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. 

Lectins, found in all organisms, are proteins that bind carbohydrates with high affinity and specificity (Table 7-3). Lectins serve in a wide variety of cell-cell recognition, signaling, and adhesion processes and in intra-... 

The conformations we have obtained are illustrated in Figs. 34 and 35, which clearly bring out the disposition in space of the N-acetyl-lac-tosamine isoglycans, and the image obtained conforms entirely with the concept that the glycans are recognition signals. In addition, the essential observations that can be extracted by examination of molecu-... 

The conformation of some glycans has been studied with the aid of molecular models. For structures of the N-acetyl-lactosaminic type, having two antennae, two conformations are possible, the Y and the T. Results from X-ray diffraction studies favor the T conformation. Both conformations are in good agreement with the role of recognition signal that the glycans could play. 

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