Cytokines 4-helix bundle

Like other hormones in this class of cytokines, GH has a four-helix bundle structure as described in Chapter 3 (see Figures 3.7 and 13.18). Two of the a helices, A and D, are long (around 30 residues) and the other two are about 10 residues shorter. Similar to other four-helix bundle structures, the internal core of the bundle is made up almost exclusively of hydrophobic residues. The topology of the bundle is up-up-down-down with two cross-over connections from one end of the bundle to the other, linking helix A with B and helix C with D (see Figure 13.18). Two short additional helices are in the first cross-over connection and a further one in the loop connecting helices C and D. 

Neupogen), a four-helix bundle cytokine, is formulated at pH 4 but has been shown to maintain both thermal stability and tertiary structure at pH 2.60 In fact, the secondary structure of this molecule was shown to remain highly helical at pH 4 (Tm approximately 62°C) and 2 (Tm approximately 63°C) as compared to pH 7 (Tm approximately 55°C) where a less conformationally stable form was observed. In the same study, FTIR and CD data corroborated the tendency of the protein to unfold as measured by the loss of helical structure in the order pH 7 > pH 4 > pH 2. Moreover, after determining optimal pH conditions of thermostability, several studies have shown that excipient screening at such conditions can successfully predict the rank of formulation cocktails that offer the most favorable stability.14 23 31 56... 

Cytokines all function using a group of transmembrane receptors embedded in the plasma membranes of target cells. The receptors have no tyrosine kinase activity but associate with and activate kinases known as Janus kinases (JAKs). These kinases phosphory-late tyrosine side chains in their receptors, and the phosphorylated receptors activate transcription factors of the STAT (signal transducer-activators of transcription) group.186-195 The specificity of cytokine action results from a combination of receptor recognition and recognition of the various STAT molecules by different JAKs.111 Cytokines have a variety of structures. Many are helix bundles or have (3 sheet structures (Fig. 30-6). 

One of the interesting questions is why this approach has not been reported to have been used to successfully identify new members of other families of cytokines, such as the four helix bundle family which includes IL-2, IL-4, IL-5, etc. One problem for these families is that the defining features are not so apparent (for example the positions of the disulfide bonds are not always conserved). Also, the majority of the members of these cytokine families are only finally confirmed once their three-dimensional structures have been solved. It may be that when more sophisticated versions of such techniques as Profile searching can be used will this then open up new cytokines for more classical families. Such Profiles would have to include amino acid similarities, as well as secondary structure propensity. Even so, the current rate of success is not expected to be as high as for the chemokine area (see, for example, ref. 15). 

The defining structural feature of the hematopoietic class of cytokines is a four-helix bundle motif organized into four anti-parallel hehces that adopt an up-up-down-down motif (Bazan, 1990a Sprang and Bazan,... 

Fig. 1. Grouping of hematopoietic cytokine receptors by shared receptor usage. The majority of the hematopoietic cytokine receptors incorporate one of three shared signaling receptors, either the common beta (/ c) chain, the common gamma (7,-) chain or gplSO. The crystal structure of the growth hormone (red) complex (inset) was the first structure of a four helix bundle cytokine in complex with its receptor (green) (de Vos et al, 1992) and established the paradigm of cytokine/receptor complex formation. (See Color Insert.)...

Fig. 3. The gplSO family of cytokines and receptors. Schematic representation of gpl30 and leukemia inhibitory factor receptor (LIFR) oriented in a cell membrane. Of the four-helix bundle gplSO cytokines, structural information currently exists for human interleukin 6 (IL-6) (green) (Somers et al, 1997), human herpes virus interleukin 6 (HlTV-8 IL-6) (purple) (Chow et al, 2001a), ciliary neurotrophic factor (CNTF) (orange) (McDonald et al., 1995), leukemia inhibitory factor (LIF) (blue) (Robinson et al, 1994), and oncostatin-M (OSM) (red) (Deller et al, 2000). Lower panel is a detailed list of gplSO cytokines and the associated receptors incorporated into the final signaling complex. (See Color Insert.)...

In a similar fashion to the classical GH system, gplSO cytokines mediate the assembly of higher-order signaling complexes through distinct epitopes. The sites I and II, originally identified in GH, formed by distinct helical faces of the four-helix bundle are conserved in the gplSO cytokines. In addition, gplSO cytokines also include a third epitope, termed site III,... 

Nicola, N. A., and Hilton, D. J. (1998). General classes and functions of four-helix bundle cytokines. Adv. Protein. Chem. 52, 1-65. 

The predominant feature of each o-helical cytokine structure is a left-handed anti-parallel four-helix bundle (Fig. 2) (Presnell and Cohen, 1989 Sprang and Bazan, 1993). The four helices of the bundle are connected by two long overhand connections and one short segment to form a distinct up-up-down-down topology first described for porcine growth hormone... 

Until recently, the contribution of staphylococcal superantigen and skin-infiltrating effector memory T cells to the development of pruritus remained an enigma. A recent study by Dillon et al. [14] demonstrated that transgenic overexpression of the novel 4-helix bundle cytokine IL-31 in lymphocytes induces severe pruritus and dermatitis in mice. IL-31 is preferentially expressed by Th2 cells and signals through a heterodimeric receptor composed of IL-31 receptor A and oncostatin M receptor [14], Subsequently, studies by Bilsborough et al. [77] and Sonkoly et al. [78] extended these observations and showed that... 

Horseman ND Yu-Lee LY (1994) Transcriptional regulation by the helix bundle peptide hormones growth hormone, prolactin, and hematopoietic cytokines. Endocrinol Rev, 15 627-649. 

Yes. Most cytokines have a common four-helix bundle structure. Experimental techniques including scanning alanine mutagenesis have been used to show that cytokines have several regions that are recognized by their receptors. Therefore, one cytokine molecule can act to cross-link two separate receptor molecules. 

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