It has recently been shown that 20 kDa human growth hormone (hGH) forms the 1:2 hGH:hGH receptor (hGHR) complex and expresses full agonistic activity, although it hardly forms the 1:1 GH:GHR complex as compared with 22 kDa hGH. To clarify this mechanism, we analyzed the mode of receptor dimerization of 20 kDa hGH using the intact form and mutants. Complex formation analysis between hGHR extracellular domain (hGHBP) and either site1 mutant (K157A) or site2 mutant (G105R) by gel-filtration showed that the site1 mutant apparently formed no 1:1 complex and that the site2 mutant formed only the 1:1 complex. Cell proliferation analysis revealed that the activity curve (vs ligand concentration) of 20 kDa hGH showed a bell-shaped pattern. This indicates that the receptor dimerization of 20 kDa hGH proceeds in a sequential manner. Based on this sequential binding we have produced a mathematical model for receptor dimerization as a function of [hGH], [hGHBP], K(d) values for the first hGHBP binding (K(d1)) and the second hGHBP binding (K(d2)). The result of 20 kDa hGH binding to (S201C) hGHBP immobilized on biosensor tip showed that the K(d1) value was 1. 6x10(-8) M. Adopting this value as a constant in the function described above, we have obtained calculative hGHR dimerization curves vs hGH concentration. Since the K(d2) value could not be experimentally determined, the curves were simulatively obtained with varied K(d2) values. The simulated curve pattern coincided with the experimental result of the cell proliferation in Ba/F3-hGHR when the value 2.5x10(-10) M was adopted as K(d2). In conclusion, although the affinity of 20 kDa hGH for the first hGHR binding is reduced to one-tenth, that for the second binding is increased ten-fold in comparison with those of 22 kDa hGH, indicating that 20 kDa hGH can be an effective hGH isoform in the presence of hGHBP.
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