Intracellular Events Transmitted Following Formation Of The Hghhghr2 Complex

In contrast to the detailed understanding of the extracellular binding events, relatively little is understood about intracellular processes following receptor dimerization. One of the key issues yet to be addressed concerns how receptor dimerization changes the structure of the intracellular domain so that it is capable of binding protein kinases and/or transcriptional elements.

The cytoplasmic region of the hGHR is 350 residues in length; presently, little is known about the structural organization of this domain. Very little overall homology is found between intracellular domains for receptors in the hematopoietic superfamily. Since no structural information is available, it remains to be seen whether common folding motifs exist in the intracellular domains of these receptors. However, most receptors in the extended family do have a proline-rich segment (box 1; residues 276-286 in hGHR) found about 10 amino acids from the transmembrane-spanning helix (29,30) and another short conserved stretch of amino acids (box 2; residues 325-339 in hGHR) further in sequence from the membrane (31) (Fig. 6). The intracellular domain of the hGHR shows no sequence homology to known protein kinase domains, however, intracellular tyrosine kinase activity associates noncovalently with ligand binding and receptor dimerization (32,33).

A number of studies using various cell lines have shown that hGH stimulates tyrosine phosphorylation of the hGHR intracellular domain, the JAK2 intracellular protein kinase, and several STAT proteins (34-36). It has also been demonstrated that JAK2 directly interacts with the hGHR (37). Following hGH-induced receptor dimerization, JAK2 rapidly phosphorylates itself as well as specific tyrosine residues on the receptor and on STAT (Fig. 6). STATs 1, 3, and 5 have all been implicated in the growth hormone signaling pathway (38,39). Even though the intracellular domain of the hGHR is also phosphorylated, mutation of the phosphotyrosine residues does not affect proliferative signaling or JAK2/STAT activation (40).

The molecular basis for the activation process and the interaction between the intra-cellular domain and downstream signaling molecules, such as JAK2 and STAT, is pres-

UGH hGH

UGH hGH

Fig. 6. Model for the activation of cytosolic components following hGH-induced receptor dimer-ization. Shown in the intracellular domain of the two hGHRs are the location of the consensus sequences referred to as box 1 and box 2. This model shows the association and activation of the JAK2 tyrosine kinase following receptor dimerization (37). This complex in turn can activate via phosphorylation the DNA transcription protein, STAT (p91). The phosphorylated and dimerized STAT protein translocates to the nucleus and binds to DNA elements that are responsible for gene transcription

Fig. 6. Model for the activation of cytosolic components following hGH-induced receptor dimer-ization. Shown in the intracellular domain of the two hGHRs are the location of the consensus sequences referred to as box 1 and box 2. This model shows the association and activation of the JAK2 tyrosine kinase following receptor dimerization (37). This complex in turn can activate via phosphorylation the DNA transcription protein, STAT (p91). The phosphorylated and dimerized STAT protein translocates to the nucleus and binds to DNA elements that are responsible for gene transcription ently not well understood. A limited number of mutagenesis and functional studies have revealed only a fraction of the information required for complete description of the activation process. First, it has been shown that both a proliferative response and JAK2/ STAT phosphorylation can be induced by hGH on a truncated version of the hGHR, which contains only the first 54 residues of the intracellular domain (31,41). In the intracellular domain of the hGHR, a short stretch of approx 25 amino acids adjacent to the membrane is absolutely required for hGH-induced activity (42). This region contains the box 1 consensus sequence. Mutation of proline residues and a lysine within or near the conserved box 1 region causes loss of hGH-induced responses (41-43). Of course, improved understanding of the roles of individual residues in the hGHR intracellular domain will come with structures of relevant molecular complexes.

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