title: Naturally occurring mutants inform SHBG structure and function.
abstract: SHBG transports and regulates the activities of androgens and estrogens. Several single nucleotide polymorphisms in the human SHBG gene have been linked to sex steroid-dependent diseases, including those associated with the metabolic syndrome. The N-terminal laminin G-like domain of SHBG includes binding sites for calcium, sex steroids, and fibulin family members, as well as a dimerization domain. We have found that 8 of 18 uncharacterized nonsynonymous single nucleotide polymorphisms within this domain alter the production or biochemical properties of SHBG in ways not previously recognized. O-Linked glycosylation at Thr7 is disrupted in SHBG T7N, whereas abnormal glycosylation of SHBG G195E limits its secretion. Three SHBG mutants (R135C, L165M, and E176K) bind estradiol with abnormally high affinity. SHBG R135C also has an increased interaction with fibulin-2. Two different substitutions within the dimer interface at R123 (R123H and R123C) reduce the affinity for 5α-dihydrotestosterone, while increasing the relative binding affinity for estradiol. SHBG T48I is defective in calcium binding, which leads to a defect in dimerization, reduced affinity for sex steroids, and an enhanced interaction with fibulin-2, which can all be restored by calcium supplementation. These naturally occurring mutants provide insight into SHBG structure and function, and defects in SHBG production or function need to be considered in the context of its utility as a biomarker of diseases.
authors.affiliation: Department of Obstetrics and Gynecology (T.-S.W., G.L.H.) and Department of Cellular and Physiological Sciences (G.L.H.), University of British Columbia, Vancouver, British Columbia, V6T 1Z3 Canada.
authors.forename: Tsung-Sheng
authors.initials: TS
authors.lastname: Wu
authors.forename: Geoffrey L
authors.initials: GL
authors.lastname: Hammond
mesh_terms: D000818:Animals; D001665:Binding Sites / Q000235:genetics; D016466:CHO Cells; D002118:Calcium / Q000378:metabolism*; D002135:Calcium-Binding Proteins / Q000378:metabolism*; D002460:Cell Line; D003412:Cricetulus; D013196:Dihydrotestosterone / Q000378:metabolism*; D004958:Estradiol / Q000378:metabolism*; D016326:Extracellular Matrix Proteins / Q000378:metabolism*; D006031:Glycosylation; D006801:Humans; D020641:Polymorphism, Single Nucleotide; D011485:Protein Binding / Q000235:genetics; D055503:Protein Multimerization; D017434:Protein Structure, Tertiary; D011956:Receptors, Cell Surface / Q000378:metabolism; D011994:Recombinant Proteins / Q000096:biosynthesis / Q000235:genetics; D016415:Sequence Alignment; D012738:Sex Hormone-Binding Globulin / Q000235:genetics* / Q000648:ultrastructure
chemical_list: D002135:Calcium-Binding Proteins; D016326:Extracellular Matrix Proteins; D011956:Receptors, Cell Surface; D011994:Recombinant Proteins; D012738:Sex Hormone-Binding Globulin; C084612:fibulin 2; C063572:sex hormone-binding globulin receptor; D013196:Dihydrotestosterone; D004958:Estradiol; D002118:Calcium