Secondary structure and shape of plasma sex steroid-binding protein--comparison with domain G of laminin results in a structural model of plasma sex steroid-binding protein.
We have analyzed the secondary structure, shape and dimensions of plasma sex steroid-binding protein (SBP) by CD, size-exclusion chromatography and electron microscopy. CD spectra show extrema at 186 nm and 216 nm characteristic for beta-sheet structures. Analysis with different algorithms indicates 15% alpha-helix, 43% beta-sheet and 10-16% beta-turn structures. An irreversible structural change is observed upon heating above 60 degrees C, which correlates with the loss of steroid-binding activity. As the SBP sequence shows similarity with domains of several multidomain proteins, including laminins, we evaluated the structure of domain G of laminin-1. The CD spectrum shows extrema at 200 nm and 216 nm. Deconvolution results in 13% alpha-helix, 32% beta-sheet and 15% beta-turn structures. Steroid-binding assays indicate that laminin and fragments thereof have no activity. Size-exclusion chromatography reveals that SBP has an extended shape and can be modeled as a cylinder with a length and diameter of 23 nm and 3 nm, respectively. This shape and the dimensions are in agreement with the appearance on electron micrographs. We propose a model for the structure of SBP in which two monomers assemble head to head with the steroid-binding site located in the center of the rod-like particle.
Beck
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K
Institute for Biophysics, Johannes Kepler University, Linz, Austria. beckho@umdnj.edu
Gruber
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TM
Ridgway
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Hughes
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Sui
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Pétra
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D000595:Amino Acid Sequence; D006801:Humans; D007797:Laminin / Q000737:chemistry*; D008854:Microscopy, Electron; D008961:Models, Structural; D008969:Molecular Sequence Data; D010446:Peptide Fragments / Q000737:chemistry / Q000378:metabolism; D017433:Protein Structure, Secondary; D012738:Sex Hormone-Binding Globulin / Q000737:chemistry*
D007797:Laminin; D010446:Peptide Fragments; D012738:Sex Hormone-Binding Globulin