Bone morphogenetic protein receptor complexes on the surface of live cells: a new oligomerization mode for serine/threonine kinase receptors

The bone morphogenetic proteins (BMPs) play important roles in embryogenesis and normal cell growth. The BMP receptors belong to the family of serine/threonine kinase receptors, whose activation has been investigated intensively for the transforming growth factor-beta (TGF-beta) receptor subfamily. However, the interactions between the BMP receptors, the composition of the active receptor complex, and the role of the ligand in its formation have not yet been investigated and were usually assumed to follow the same pattern as the TGF-beta receptors. Here we demonstrate that the oligomerization pattern of the BMP receptors is different and is more flexible and susceptible to modulation by ligand. Using several complementary approaches, we investigated the formation of homomeric and heteromeric complexes between the two known BMP type I receptors (BR-Ia and BR-Ib) and the BMP type II receptor (BR-II). Coimmunoprecipitation studies detected the formation of heteromeric and homomeric complexes among all the BMP receptor types even in the absence of ligand. These complexes were also detected at the cell surface after BMP-2 binding and cross-linking. Using antibody-mediated immunofluorescence copatching of epitope-tagged receptors, we provide evidence in live cells for preexisting heteromeric (BR-II/BR-Ia and BR-II/BR-Ib) and homomeric (BR-II/BR-II, BR-Ia/ BR-Ia, BR-Ib/ BR-Ib, and also BR-Ia/ BR-Ib) oligomers in the absence of ligand. BMP-2 binding significantly increased hetero- and homo-oligomerization (except for the BR-II homo-oligomer, which binds ligand poorly in the absence of BR-I). In contrast to previous observations on TGF-beta receptors, which were found to be fully homodimeric in the absence of ligand, the BMP receptors show a much more flexible oligomerization pattern. This novel feature in the oligomerization mode of the BMP receptors allows higher variety and flexibility in their responses to various ligands as compared with the TGF-beta receptors.

Transforming growth factor-beta induces formation of a dithiothreitol-resistant type I/Type II receptor complex in live cells

Transforming growth factor-beta (TGF-beta) binds to and signals via two serine-threonine kinase receptors, the type I (TbetaRI) and type II (TbetaRII) receptors. We have used different and complementary techniques to study the physical nature and ligand dependence of the complex formed by TbetaRI and TbetaRII. Velocity centrifugation of endogenous receptors suggests that ligand-bound TbetaRI and TbetaRII form a heteromeric complex that is most likely a heterotetramer. Antibody-mediated immunofluorescence co-patching of epitope-tagged receptors provides the first evidence in live cells that TbetaRI. TbetaRII complex formation occurs at a low but measurable degree in the absence of ligand, increasing significantly after TGF-beta binding. In addition, we demonstrate that pretreatment of cells with dithiothreitol, which inhibits the binding of TGF-beta to TbetaRI, does not prevent formation of the TbetaRI.TbetaRII complex, but increases its sensitivity to detergent and prevents TGF-beta-activated TbetaRI from phosphorylating Smad3 in vitro. This indicates that either a specific conformation of the TbetaRI. TbetaRII complex, disrupted by dithiothreitol, or direct binding of TGF-beta to TbetaRI is required for signaling.

Oligomeric structure of type I and type II transforming growth factor beta receptors: homodimers form in the ER and persist at the plasma membrane

Transforming growth factor beta (TGF-beta) signaling involves interactions of at least two different receptors, types I (TbetaRI) and II (TbetaRII), which form ligand-mediated heteromeric complexes. Although we have shown in the past that TbetaRII in the absence of ligand is a homodimer on the cell surface, TbetaRI has not been similarly investigated, and the site of complex formation is not known for either receptor. Several studies have indicated that homomeric interactions are involved in TGF-beta signaling and regulation, emphasizing the importance of a detailed understanding of the homooligomerization of TbetaRI or TbetaRII. Here we have combined complementary approaches to study these homomeric interactions in both naturally expressing cell lines and cells cotransfected with various combinations of epitope-tagged type I or type II receptors. We used sedimentation velocity of metabolically labeled receptors on sucrose gradients to show that both TbetaRI and TbetaRII form homodimer-sized complexes in the endoplasmic reticulum, and we used coimmunoprecipitation studies to demonstrate the existence of type I homooligomers. Using a technique based on antibody-mediated immunofluorescence copatching of receptors carrying different epitope tags, we have demonstrated ligand-independent homodimers of TbetaRI on the surface of live cells. Soluble forms of both receptors are secreted as monomers, indicating that the ectodomains are not sufficient to mediate homodimerization, although TGF-beta1 is able to promote dimerization of the type II receptor ectodomain. These findings may have important implications for the regulation of TGF-beta signaling.

Roles for a cytoplasmic tyrosine and tyrosine kinase activity in the interactions of Neu receptors with coated pits

The neu proto-oncogene product, p185neu (HER2, c-ErbB-2), encodes a cell-surface tyrosine kinase receptor with high oncogenic potential, which correlates with increased tyrosine kinase activity and a rapid receptor internalization rate. To investigate the interactions and signal(s) leading to the endocytosis of Neu receptors, we employed lateral mobility and internalization studies. Fluorescence photobleaching recovery measurements revealed that activation of Neu receptors (induced by mutation or by agonistic antibodies) markedly reduced their mobile fractions. To elucidate the signals involved, other mutants, all carrying a constitutively dimerizing oncogenic mutation, were analyzed. A kinase-negative mutant and a mutant lacking all cytoplasmic tyrosine phosphorylation consensus sequences exhibited high mobile fractions, similar to nonactivated Neu. Retention of a single tyrosine autophosphorylation site (Tyr-1253) out of the five known such sites was sufficient to immobilize a large fraction of the receptor. For all mutants, internalization correlated with receptor immobilization and was blocked by treatments that interfere with coated pit structure, indicating that the immobilization is due to interactions with coated pits. This was supported by the coimmunoprecipitation of alpha-adaptin only with the constitutively activated Neu mutants. We conclude that activated Neu receptors become stably associated with coated pits via plasma membrane adaptor complexes (AP-2). Efficient Neu receptor endocytosis requires activation, a functional kinase domain, and at least one tyrosine autophosphorylation site.

The epidemiology of demoralization in a kibbutz

This article reports a true prevalence study of demoralization conducted in a kibbutz. This commune, which assures its members all their instrumental needs throughout the individual life span, is probably one of the few of its kind in the world. All adult members (n = 353) of a kibbutz affiliated with the most orthodox of the federations replied to a self-administered interview containing the Psychiatric Epidemiology Research interview 27-item Demoralization Scale. The response rate approached 95%. Univariate and multivariate analyses were conducted using demoralization rates and mean scores as dependent variables. The overall prevalence of demoralization was 25%. Women exhibited higher rates and mean scores. There was a clear association between occupational prestige scores and demoralization in both sexes. Discussion is based on the comparison between the study findings and those published in the literature, both in terms of the overall results and their pattern of association with key sociodemographic variables.