Threonine 788 in integrin subunit beta1 regulates integrin activation

In the present study, the functional role of suggested phosphorylation of the conserved threonines in the cytoplasmic domain of integrin subunit beta1 was investigated. Mutants mimicking phosphorylated and unphosphorylated forms of beta1 were expressed in beta1 deficient GD25 cells. T788 in beta1 was identified as a site with major influence on integrin function. The mutation to A788 strongly reduced beta1-dependent cell attachment and exposure of the extracellular 9EG7 epitope, whereas replacement of T789 with alanine did not interfere with the ligand-binding ability. Talin has been shown to mediate integrin activation, but the talin head domain bound equally well to the wild-type beta1 and the mutants indicating that the T788A mutation caused defect integrin activation by another mechanism. The phosphorylation-mimicking mutation T788D was fully active in promoting cell adhesion. GD25 cells expressing beta1T788D accumulated increased number of focal contacts and migrated slowly compared to GD25 beta1 wild-type. An analogous phenotype is seen when focal adhesion kinase activation is abrogated. However, neither the beta1T788D nor the beta1T788A mutation failed to induce tyrosine phosphorylation of focal adhesion kinase. The results suggest that phosphorylation of T788 in integrin beta1 promotes inside-out receptor activation, as well as focal contact accumulation.     

Nucleotide sequence of cloned cDNA coding for pumpkin 11-S globulin beta subunit

cDNA coding for preproglobulin beta, a precursor protein of 11-S globulin beta subunit, was cloned and the nucleotide sequence has been determined. The sequence covers the whole coding region (1440 base pairs) with 5' and 3' noncoding region (30 and 214 base pairs, respectively). The deduced amino acid sequence of preproglobulin beta consists of a 21-amino-acid N-terminal signal peptide, preceding the acidic gamma polypeptide region (275 amino acids) and the subsequent basic delta region (184 amino acids). The site for post-translational cleavage of the precursor polypeptide to make the gamma and delta chains is estimated to be located between the asparagine-glycine residues. The N-terminal amino acid of the gamma chain of mature 11-S globulin beta subunit was reported to be blocked by 5-oxoproline (pyroglutamic acid) [Ohmiya et al. (1980) Plant Cell Physiol. 21, 157-167]. It was shown that the blocked N-terminal amino acid is coded as a glutamine residue. The derived amino acid sequence was also compared with those of precursor proteins of other 11-S globulins such as soybean glycinin, cotton beta globulin, pea legumin and rape 11-S globulin by dot matrix analysis.     

Activation of the alpha 4 beta 1 integrin through the beta 1 subunit induces recognition of the RGDS sequence in fibronectin

Lymphocyte attachment to fibronectin is mainly mediated by the interaction of alpha 5 beta 1 and alpha 4 beta 1 integrins with the RGD and CS-1/Hep II sites, respectively. We have recently shown that the anti-beta 1 mAb TS2/16 can convert the partly active alpha 4 beta 1 present on certain hemopoietic cells that recognizes CS-1 but not Hep II, to a high avidity form that binds both ligands. In this report we have studied whether mAb TS2/16 also affects alpha 4 beta 1 ligand specificity. Incubation of the B cell lines Ramos and Daudi (which lack alpha 5 beta 1) with mAb TS2/16 induced specific attachment to an 80-kD fragment which lacks CS-1 and Hep II and contains the RGD sequence. mAbs anti-alpha 4 and the synthetic peptides CS-1 and IDAPS inhibited adhesion to the 80-kD fragment thus implying alpha 4 beta 1 as the receptor for this fragment. Interestingly, the synthetic peptide GRGDSPC and a 15-kD peptic fibronectin fragment containing the RGD sequence also inhibited B cell adhesion to the 80-kD fragment. Because we have previously shown that RGD peptides do not affect the constitutive function of alpha 4 beta 1, we tested whether TS2/16- activated alpha 4 beta 1 acquired the capacity to recognize RGD. Indeed RGD peptides inhibited TS2/16-treated B cell adhesion to a 38-kD fragment containing CS-1 and Hep II but did not affect binding of untreated cells to this fragment. An anti-fibronectin mAb reactive with an epitope on or near the RGD sequence also efficiently inhibited cell adhesion to the 80-kD fragment, indicating that the RGD sequence is a novel adhesive ligand for activated alpha 4 beta 1. These results emphasize the role of alpha 4 beta 1 as a receptor with different ligand specificities according to the activation state, a fact that may be important for lymphocyte migration, localization, and function.     

Annexin-V binds to the intracellular part of the beta(5) integrin receptor subunit

Bovine lactadherin binds to the alpha(v)beta(3) and alpha(v)beta(5) integrins in an RGD-dependent manner and also to anionic phospholipids. During the affinity purification of lactadherin binding receptors, a 35-kDa protein persistently coeluted with the alpha(v)beta(5) integrin receptor. Subsequently, peptide mapping, amino acid sequencing, and mass spectrometry analysis identified this protein as bovine annexin-V. Annexin-V accompanied the integrin receptor eluted with either RGD peptide or with EDTA suggesting that annexin-V bound specifically to the alpha(v)beta(5) integrin. To further investigate this putative interaction of annexin-V with the alpha(v)beta(5) integrin receptor, human annexin-V and intracellular domains of the human alpha(v)beta(5) integrin subunits were used in ligand blotting assays. Radiolabeled annexin-V showed weak binding to the intracellular part of beta(5) integrin subunit. However, by adding the aminophospholipid, phosphatidyl serine, the interaction with the beta(5) cytoplasmic peptide was enhanced many fold. Furthermore, the interaction was shown to be independent of phosphorylation, as annexin-V bound to unphosphorylated beta(5) peptide at a similar level to the phosphorylated peptide. Since binding of annexin-V to the alpha(v) integrin subunit tail was not detected, annexin-V was shown to associate specifically with the beta(5) cytoplasmic tail. Together these findings suggest a novel link between annexins and the integrin receptor family.     

cDNA sequence of the beta 2-subunit of human liver alcohol dehydrogenase

A cDNA library of mRNA from a human liver expressing the beta 2-subunit of alcohol dehydrogenase was constructed in lambda gt11. One clone coding for 352 of a total of 374 amino acid residues of the beta 2-subunit was isolated. The sequence differed from that of the beta 1-subunit at one nucleotide position resulting in an Arg/His exchange at position 47 of the peptide chain, in agreement with data from protein sequence analysis [(1984) FEBS Lett. 173, 360-366].     

A highly conserved sequence of the Arg-Gly-Asp-binding domain of the integrin beta 3 subunit is sensitive to stimulation

The Arg-Gly-Asp (RGD)-binding domain of GPIIb-IIIa has been localized in a fragment of the GPIIIa subunit that includes the sequence between amino acids 109 and 171. To examine, in a platelet membrane environment, the activated versus nonactivated status of this domain, we have produced a monoclonal antibody against a synthetic peptide (residues 109-128) located within the RGD-binding region on GPIIIa. This kappa-IgM, named AC7, was specific for GPIIIa peptide 109-128 and interacted only with activated platelets. Fibrinogen, RGDF peptide, and the fibrinogen phi chain decapeptide LGGAKQAGDV inhibited the binding of AC7 to ADP-stimulated platelets. AC7 IgM and "small fragments" inhibited fibrinogen binding and platelet aggregation in a dose-dependent fashion. Induction of AC7 binding by D33C, a monoclonal antibody recognizing the GPIIb 426-437 sequence and stimulating fibrinogen binding, indicated that the GPIIb 426-437 and the GPIIIa 109-128 sequences were both involved in a stimulation-dependent conformational modification of the receptor. AC7 was able to recognize beta subunits other than GPIIIa on leucocyte surfaces but only after cell fixation with glutaraldehyde. The results are consistent with the implication of the RGD-binding domain in receptor ligand interaction on the platelet surface and its conformational modification and exposure upon receptor induction.     

Structure-function of the putative I-domain within the integrin beta 2 subunit

The central region (residues 125-385) of the integrin beta(2) subunit is postulated to adopt an I-domain-like fold (the beta(2)I-domain) and to play a critical role in ligand binding and heterodimer formation. To understand structure-function relationships of this region of beta(2), a homolog-scanning mutagenesis approach, which entails substitution of nonconserved hydrophilic sequences within the beta(2)I-domain with their homologous counterparts of the beta(1)I-domain, has been deployed. This approach is based on the premise that beta(1) and beta(2) are highly homologous, yet recognize different ligands. Altogether, 16 segments were switched to cover the predicted outer surface of the beta(2)I-domain. When these mutant beta(2) subunits were transfected together with wild-type alpha(M) in human 293 cells, all 16 beta(2) mutants were expressed on the cell surface as heterodimers, suggesting that these 16 sequences within the beta(2)I-domain are not critically involved in heterodimer formation between the alpha(M) and beta(2) subunits. Using these mutant alpha(M)beta(2) receptors, we have mapped the epitopes of nine beta(2)I-domain specific mAbs, and found that they all recognized at least two noncontiguous segments within this domain. The requisite spatial proximity among these non-linear sequences to form the mAb epitopes supports a model of an I-domain-like fold for this region. In addition, none of the mutations that abolish the epitopes of the nine function-blocking mAbs, including segment Pro(192)-Glu(197), destroyed ligand binding of the alpha(M)beta(2) receptor, suggesting that these function-blocking mAbs inhibit alpha(M)beta(2) function allosterically. Given the recent reports implicating the segment equivalent to Pro(192)-Glu(197) in ligand binding by beta(3) integrins, these data suggest that ligand binding by the beta(2) integrins occurs via a different mechanism than beta(3). Finally, both the conformation of the beta(2)I-domain and C3bi binding activity of alpha(M)beta(2) were dependent on a high affinity Ca(2+) binding site (K(d) = 105 microm), which is most likely located within this region of beta(2).     

Cloning, primary structure and properties of a novel human integrin beta subunit.

The originally described integrin beta subunits that define the three subfamilies of integrin heterodimers are beta 1, beta 2 and beta 3. In this paper, we describe the isolation of a cDNA coding for a novel human integrin beta subunit, designated as beta 5. The beta 5 cDNA was isolated from a human thymic epithelial cell library, using oligonucleotide probes that were designed from a region highly conserved among the known beta 1, beta 2 and beta 3 sequences. The beta 5 cDNA codes for 799 (or 796) amino acids, including a 23 amino acid leader sequence. There are 776 (or 773) amino acids in the mature protein, which includes a long extracellular domain of 696 amino acids, a transmembrane domain and an intracellular C-terminal domain of 57 amino acids. The beta 5 sequence resembled the known beta 3, beta 1 and beta 2 sequences by 55, 43 and 38%, respectively, including conservation of 56/56 cysteines. Rabbit antiserum was prepared against a 20 amino acid synthetic peptide predicted from the beta 5 C-terminal sequence. This serum immunoprecipitated a beta 5 protein that was 100,000 Mr (reduced) and 95,000 Mr (nonreduced). Only a single alpha subunit was detected in association with beta 5, and that alpha subunit was immunochemically indistinguishable from the alpha v subunit previously found as part of the vitronectin receptor complex. By immunoprecipitation, beta 5 was most prevalent on carcinoma cell lines, was also present on hepatoma and fibroblast cell lines, and was absent from lymphoblastoid cells and platelets.     

Structure and sequence analysis of the human activin beta A subunit gene.

The cloned genomic DNA containing the human activin beta A subunit gene were analyzed by restriction endonuclease mapping, Southern blotting and DNA sequencing. The activin gene is composed of two exons interrupted by the 9-kb intron. The TATA, CCAAT and CT-stretch sequences were found in the 5'-flanking region of the gene. An intronic sequence contained SV40 enhancer core element in the vicinity of the exon 1. In the 3'-flanking region, we identified eight consensus polyadenylation sequences, five ATTTA motifs, CA element consisting of (CA)14, AP-1 binding site and two SV40 enhancer core elements. A dot matrix analysis revealed the high degree of conservation between the human and rat sequences within the 3'-flanking region, suggesting a possible functional significance.