title: A novel missense mutation shows that GPIbbeta has a dual role in controlling the processing and stability of the platelet GPIb-IX adhesion receptor.
abstract: Glycoprotein (GP) Ibalpha is a major adhesive receptor of platelets, surface expressed as part of the GPIb-IX-V complex. However, important questions about how the four gene products (Ibalpha, Ibbeta, IX, and V) composing this complex are processed remain. A deficiency of or nonfunctioning GPIb-IX-V is characteristic of the Bernard-Soulier syndrome (BSS), an inherited bleeding disease. We now report a BSS variant whose platelets have little or no GIbbeta or GPIX, but where residual GPIbalpha was selectively located in flow cytometry by monoclonal antibodies (WM23 and Bx-1) recognizing denatured epitopes. Whereas WM23 immunoprecipitated GPIbalpha (130 kDa), GPIX, and GPIbbeta from control platelets, a single surface protein of approximately 66 kDa was obtained for the patient. DNA sequencing revealed a homozygous Asn(64) --> Thr substitution in the GPIbbeta from the patient. This substitution modified a conserved residue in the COOH-terminal region flanking the single-copy leucine-rich domain of GPIbbeta. When GPIbbeta64Thr was coexpressed in a stable CHO cell line with wild-type GPIbalpha and GPIX, flow cytometry and confocal microscopy failed to show GPIb-IX complexes at the cell surface. Intracellular GPIbalpha and GPIbbeta were detected and largely confined to the endoplasmic reticulum, and little GPIX was seen. GPIbalpha was immunoprecipitated as a 66-70 kDa protein in (35)S metabolic studies and lacked O-glycosidic side chains. Also, it was not disulfide bound to the mutated GPIbbeta. Thus, a single amino acid substitution in the extracellular domain of GPIbbeta can affect both the maturation of GPIbalpha and GPIX stability. GPIbbeta has a pivotal role in regulating GPIb-IX-V biosynthesis.
authors.affiliation: INSERM U311-Etablissement Français du Sang, Strasbourg, France.
authors.forename: Catherine
authors.initials: C
authors.lastname: Strassel
authors.forename: Jean-Max
authors.initials: JM
authors.lastname: Pasquet
authors.forename: Marie-Christine
authors.initials: MC
authors.lastname: Alessi
authors.forename: Irène
authors.initials: I
authors.lastname: Juhan-Vague
authors.forename: Hervé
authors.initials: H
authors.lastname: Chambost
authors.forename: Robert
authors.initials: R
authors.lastname: Combrié
authors.forename: Paquita
authors.initials: P
authors.lastname: Nurden
authors.forename: Marie-Jeanne
authors.initials: MJ
authors.lastname: Bas
authors.forename: Corinne
authors.initials: C
authors.lastname: De La Salle
authors.forename: Jean-Pierre
authors.initials: JP
authors.lastname: Cazenave
authors.forename: François
authors.initials: F
authors.lastname: Lanza
authors.forename: Alan T
authors.initials: AT
authors.lastname: Nurden
mesh_terms: D000293:Adolescent; D000818:Animals; D019921:Biotinylation; D001792:Blood Platelets / Q000378:metabolism; D016466:CHO Cells; D006224:Cricetinae; D005434:Flow Cytometry; D005455:Fluorescent Antibody Technique; D006801:Humans; D008297:Male; D020125:Mutation, Missense; D019038:Platelet Glycoprotein GPIb-IX Complex / Q000235:genetics* / Q000378:metabolism*; D010980:Platelet Membrane Glycoproteins; D011233:Precipitin Tests; D017422:Sequence Analysis, DNA
chemical_list: D019038:Platelet Glycoprotein GPIb-IX Complex; D010980:Platelet Membrane Glycoproteins; C032193:glycoprotein receptor GPIb-IX