Journal
JOURNAL OF MOLECULAR BIOLOGY
Volume 358, Issue 3, Pages 773-784Publisher
ACADEMIC PRESS LTD- ELSEVIER SCIENCE LTD
DOI: 10.1016/j.jmb.2006.02.038
Keywords
human placental lactogen; X-ray crystallography; human prolactin receptor; surface plasmon resonance; alanine-scanning
Categories
Funding
- NCRR NIH HHS [RR07707] Funding Source: Medline
- NIDDK NIH HHS [DK-61602] Funding Source: Medline
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In primates, placental lactogen (PL) is a pituitary hormone with fundamental roles during pregnancy involving fetal growth, metabolism, and stimulating lactation in the mother. Human placental lactogen (hPL) is highly conserved with human growth hormone (hGH) and both hormones bind to the hPRLR extracellular domain (ECD), the first step in receptor homodimerization, in a Zn2+-dependent manner. A modified surface plasmon resonance method was developed to measure the kinetics for hPL and hGH binding to the hPRLR ECD, with and without Zn2+ and showed that hPL has about a tenfold higher affinity for the hPRLR ECD1 than hGH. The crystal structure of the free state of hPL has been determined to 2.0 angstrom resolution showing the molecule possesses an overall structure similar to other long chain four-helix bundle cytokines. Comparison of the free hPL structure with the 1:1 complex structure of hGH bound to the hPRLR ECD1 suggests that two surface loops undergo conformational changes > 10 angstrom upon binding. An 18 residue Ala-scan was used to characterize the binding energy epitope for the site I interface of hPL. Individual alanine substitutions at five positions reduced binding affinity by a Delta Delta G >= 3 kcal mol(-1). A comparison of the hPL site I epitope with that previously determined for hGH indicates contributions of individual residues track reasonably well between hPL and hGH. In particular, residues involved in the zinc-binding site and Lys172 constitute the principal binding determinants for both hormones. However, several residues that are identical between hPL and hGH contribute quite differently to the binding of the hPRLR ECD1. Additionally, the overall magnitudes of the Delta Delta G changes observed from the Ala-scan of hPL were markedly larger than those determined in the comparative scan of hGH to the hPRLR ECD1. The structural and biophysical data presented here show that subtle changes in the structural context of an interaction can lead to significantly different effects at the individual residue level. (c) 2006 Elsevier Ltd. All rights reserved.
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