Solution conformation of the antibody-bound tyrosine phosphorylation site of the nicotinic acetylcholine receptor beta-subunit in its phosphorylated and nonphosphorylated states

Phosphorylation of the acetylcholine receptor (AChR) seems to be responsible for triggering several effects including its desensitization and aggregation at the postsynaptic membrane and probably initiates a signal transduction pathway at the postsynaptic membrane. To study the structural and functional role of the tyrosine phosphorylation site of the AChR beta-subunit and contribute to the in-depth understanding of the structural basis of the ion channel function, we synthesized four peptides containing the phosphorylated and nonphosphorylated sequences (380-391) of the human and Torpedo AChR beta-subunits and studied their interaction with a monoclonal antibody (mAb 148) that is known to bind to this region and that is capable of blocking ion channel function. All four peptides were efficient inhibitors of mAb 148 binding to AChR, although the nonphosphorylated human peptide was considerably less effective than the three others. We then investigated the conformation acquired by all four peptides in their antibody-bound state, which possibly illustrates the local conformation of the corresponding sites on the intact AChR molecule. The phosphorylated human and Torpedo peptides adopted a distorted 3(10) helix conformation. The nonphosphorylated Torpedo peptide, which is also an efficient inhibitor, was also folded. In contrast, the nonphosphorylated human peptide (a less efficient inhibitor) presented an extended structure. It is concluded that the phosphorylation of the AChR at its beta-subunit Tyr site leads to a significant change in its conformation, which may affect several functions of the AChR.

Association of acetylcholine receptor alpha-subunit gene expression in mixed thymoma with myasthenia gravis

OBJECTIVE

To investigate the association of MG with the transcription of muscular or neuronal acetylcholine receptor (AChR) subunit genes in thymomas.

BACKGROUND

Many steps in the pathogenesis of MG have been elucidated but, with rare exceptions, its etiology is unknown. In patients with MG with thymoma, the tumor probably elicits autoimmunity to AChR, but it is enigmatic why MG develops in some patients but not in others.

METHODS

Reverse transcriptase (RT)-PCR, immunohistochemistry, and immunofluorescence studies were carried out to investigate AChR expression in 35 patients with thymoma. Statistical analysis was used to specify significant differences between thymoma subtypes.

RESULTS

Considering all thymomas (n = 35), no correlation was found between MG status and AChR gene expression as detected by RT-PCR. However, when histologically defined thymoma subtypes were studied separately, transcription of the muscular AChR P3A- alpha-subunit gene was significantly associated (alpha < 0.01) with the occurrence of MG in mixed thymomas (n = 17), but not in thymomas of the cortical type. For the other muscular AChR subunits (P3A+ alpha isoform, beta, gamma, delta, and epsilon) and the alpha2 and beta4 neuronal AChR subunits, no such correlation was detected.

CONCLUSIONS

Expression of the P3A AChR alpha-subunit gene might be important for the pathogenesis of MG in mixed thymomas, suggesting etiologic heterogeneity of paraneoplastic MG among patients with histologically different thymoma subtypes.

Phosphorylation of two regulatory tyrosine residues in the activation of Bruton's tyrosine kinase via alternative receptors

Mutation of Bruton's tyrosine kinase (Btk) impairs B cell maturation and function and results in a clinical phenotype of X-linked agammaglobulinemia. Activation of Btk correlates with an increase in the phosphorylation of two regulatory Btk tyrosine residues. Y551 (site 1) within the Src homology type 1 (SH1) domain is transphosphorylated by the Src family tyrosine kinases. Y223 (site 2) is an autophosphorylation site within the Btk SH3 domain. Polyclonal, phosphopeptide-specific antibodies were developed to evaluate the phosphorylation of Btk sites 1 and 2. Crosslinking of the B cell antigen receptor (BCR) or the mast cell Fcepsilon receptor, or interleukin 5 receptor stimulation each induced rapid phosphorylation at Btk sites 1 and 2 in a tightly coupled manner. Btk molecules were singly and doubly tyrosine-phosphorylated. Phosphorylated Btk comprised only a small fraction (</=5%) of the total pool of Btk molecules in the BCR-activated B cells. Increased dosage of Lyn in B cells augmented BCR-induced phosphorylation at both sites. Kinetic analysis supports a sequential activation mechanism in which individual Btk molecules undergo serial transphosphorylation (site 1) then autophosphorylation (site 2), followed by successive dephosphorylation of site 1 then site 2. The phosphorylation of conserved tyrosine residues within structurally related Tec family kinases is likely to regulate their activation.