Disease relevance of GP1BB

• Bernard-Soulier syndrome Kagoshima: Ser 444-->stop mutation of glycoprotein (GP) Ib alpha resulting in circulating truncated GPIb alpha and surface expression of GPIb beta and GPIX [1].

High impact information on GP1BB

• While the GPIb alpha mRNA and protein appear similar in platelets and EC, EC GPIb beta mRNA is larger than platelet GPIb beta and encodes a larger protein [2].
• Complementary DNA cloning of the alternatively expressed endothelial cell glycoprotein Ib beta (GPIb beta) and localization of the GPIb beta gene to chromosome 22 [2].
• Using human hamster hybrids, we have localized the GPIb beta gene to chromosome 22pter-->22q11 [2].
• We have cloned and sequenced EC GPIb beta cDNA and report a 2793-nucleotide sequence which contains a 411-amino acid open reading frame [2].
• A putative cooperation between the GPIb alpha and GPIb beta cytoplasmic domains was investigated by measuring the adhesion under flow to immobilized VWF of K562 and Chinese hamster ovary (CHO) cells transfected with GPIb/(V)/IX containing mutations in this region [3].

Chemical compound and disease context of GP1BB

• The existence of Bernard-Soulier syndrome, a hereditary disorder in which platelets selectively lack the GP Ib.IX complex, enabled us to examine whether the phosphorylation of GP Ib beta (P24) is responsible for any of the inhibitory effects of elevated cyclic AMP on platelet function [4].

Biological context of GP1BB

• 1. In contrast, forskolin or PGE(1) treatment increased both the phosphorylation of GPIb beta and adhesion of control cells, both effects being reversed by RAM.1, but had no influence on cells expressing the Ser166Gly mutation [3].
• Based on sequence homology to the known crystal structures of two other leucine-rich repeat proteins, the human Nogo receptor and GPIb alpha, we propose a new structural model of GPIb beta [5].
• These findings indicated that null expression of the abnormal GPIb beta causes decreased expression of the complex and results in the GPD phenotype in the patient, and suggested that homozygosity of the mutation may lead to a BSS phenotype in vivo [6].
• We characterize here a novel missense mutation of the GPIb beta gene associated with isolated GPD [6].
• Transient transfection studies demonstrated that mutant GPIb beta protein was not detected in transfected 293T cells [6].

Anatomical context of GP1BB

• Both GPIb alpha and GPIb beta are expressed in endothelial cells (EC) [2].
• The present report describes a monoclonal antibody (RAM.1) that labeled the 26 kDa GPIbbeta subunit on western blots and coprecipitated the three subunits of the GPIb/IX complex from lysates of platelets and transfected CHO and K562 cells [7].
• The GPIb beta gene has a simple structure, similar to that of other described megakaryocyte/platelet genes, including those of the GPIb-V-IX system [8].
• Substitution of either Cys5 or Cys7 by Ala failed to show surface expression of GPIb-IX, suggesting that the Cys5- Cys7 disulfide loop in GPIbbeta is essential for the efficient processing and trafficking of GPIb-IX complexes toward the plasma membrane [9].
• The membrane-proximal GPIbbeta sequence was analogous to a previously reported calmodulin-binding sequence in the leukocyte adhesion receptor, L-selectin [10].

Associations of GP1BB with chemical compounds

• DNA sequencing revealed a C to T transition in the patient's remaining GP1BB allele, predicting a novel proline to serine substitution (Pro96Ser) in the carboxyterminal flanking domain of a leucine-rich repeat [5].
• Adhesion of cells carrying a glycine substitution of the GPIb beta Ser166 phosphorylation site was 50% lower than normal and became insensitive to inhibition by RAM [3].
• The cysteine knot of platelet glycoprotein Ib beta (GPIb beta) is critical for the interaction of GPIb beta with GPIX [11].
• DNA sequencing analysis revealed a novel heterozygous missense mutation in the GPIb beta gene that converts Arg (CGC) to Cys (TGC) at residue 17 [6].
• Analysis of the genes for two other components of the platelet GPIb:IX complex, namely GPIb beta and GPIX, showed two different missense mutations in the coding region of the GPIX gene: an A-->G transition in codon 21 results in conversion of an aspartic acid to glycine and an A-->G change in codon 45 converts an asparagine residue to serine [12].

Regulatory relationships of GP1BB

• Thus, PKA-mediated phosphorylation of GPIbbeta at Ser(166) negatively regulates VWF binding to GPIb-IX and is one of the mechanisms by which PKA mediates platelet inhibition [13].
• The adhesion was specifically inhibited by an anti-GPIbalpha blocking antibody (AK2) and was not observed with CHO cells expressing GPIbbeta and GPIX alone [14].
• METHODS: The residue A156 of GP Ib alpha was converted to a valine and the mutant expressed in CHO cells expressing wild-type GP Ib beta and GPIX [15].

Other interactions of GP1BB

• This occurs through the interaction of GPIX with GPIb beta [11].
• The model refutes earlier assumptions about cysteine-cysteine interactions in the amino-terminal region of GPIb beta, and predicts a hydrophobic patch the burial of which may contribute to proper conformation of the fully assembled vWF receptor complex [5].
• The complex is composed of four membrane-spanning glycoproteins (GP): GPIbalpha, GPIbbeta, GPIX, and GPV [16].
• The HCDCREL-1 gene possesses a non-consensus polyadenylation signal that apparently is not efficiently utilized, resulting in the expression of a readthrough transcript also containing the platelet GPIb beta coding region [17].

Analytical, diagnostic and therapeutic context of GP1BB

• Fluorescence in situ hybridization in transformed lymphoblasts revealed hemizygous microdeletion of 22q11.2 containing the GP1BB locus [5].
• Flow cytometry and confocal imaging of transfected CHO alpha IX cells demonstrated that P96S GPIb beta abrogates surface assembly of the complex, consistent with platelet flow cytometry studies in the patient [5].
• There were no GPIX or GPIbbeta found by immunoblotting [18].
• We analyzed all coding regions of GPIb alpha, GPIb beta, GPV and GPIX which were amplified from the patients' genomic DNA by the polymerase chain reaction (PCR) [19].
• Similarly, immunoprecipitation of GP V from cells expressing GP Ib alpha led to coprecipitation of the latter polypeptide; neither GP Ib beta nor GP IX coprecipitated with GP V from CHO beta IX cells [20].

References