Changes in ligand binding ability of the integrin alpha IIb beta 3 can be monitored by the concomitant expression of ligand-inducible binding sites (LIBS). A new LIBS, the hexapeptide sequence GPNICT (residues 1-6) at the amino terminus of beta 3 recognized by the murine monoclonal antibody (mAb) AP5, is sensitive both to the binding of ligand and to micromolar differences in divalent cation levels. Calcium or magnesium can completely inhibit the binding of AP5 to alpha IIb beta 3 on platelets, with ID50 values of 80 and 1500 microM, respectively. The inhibitory effect of calcium plus magnesium is cumulative. In the presence of 1 mM calcium plus 1 mM magnesium, the peptide RGDW overcomes this inhibition and induces maximal binding of AP5. Maximal AP5 binding is also induced by a molar excess of EDTA. The unique location of the AP5 LIBS was determined by comparing the binding of LIBS-specific mAb to recombinant human-Xenopus beta 3 chimeras produced in a baculovirus expression system. AP5 defines one region at the amino terminus beta 3 1-6. A second region, defined by mAb D3GP3, is probably located within beta 3 422-490, confirming the finding of Kouns et al. (Kouns, W. C., Newman, P.J., Puckett, K. J., Miller, A. A., Wall, C. D., Fox, C. F., Seyer, J. M., and Jennings, L. K. (1991) Blood 78, 3215-3223). The third region, encompassing at most residues 490-690, and perhaps more precisely located within 602-690 (Du X., Gu, M., Weise, J. W., Nagaswami, C., Bennett, J. S., Bowditch, R., and Ginsberg, M. H. (1993) J. Biol. Chem. 268, 23087-23092), is recognized by the four mAb, anti-LIBS2, anti-LIBS3, anti-LIBS6, and P41. Since its exposure is uniquely regulated by both divalent cations and ligand, the amino terminus of beta 3 may be involved in control of ligand binding by divalent cation mobilization.

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The components for termination of DNA replication in Escherichia coli include the terminator signals Ti and T2 and the trans-acting gene tus. We have shown previously that tus maps in a 4-kilobase region of the chromosomal terminus near T2. Through the use of deletion and insertion mutants, the location of the tus gene has now been precisely identified. We sequenced 2416 nucleotides in this region and identified a 927-base-pair open reading frame which encodes Tus. Insertion of a kanamycin-resistance gene in this open reading frame abolished tus activity. We also demonstrated that crude extracts of tusk cells contain a protein which binds to the 72 terminator sequence.Two polar terminator signals which inhibit the progress of replication forks, designated Ti and T2, have been identified in the terminus region the Escherichia coli chromosome (1, 2). Ti is located at min 28.1 (3) or kilobase (kb) 90 on the Bouche terminus map (4), and it halts only counterclockwisetraveling replication forks, whereas 72 maps at min 35.6 (3, 5) or kb 442, and it halts only clockwise-traveling replication forks. In addition to Ti and 72, terminator signals have also been identified in the E. coli chromosome near min 34,t at min 27 (J.-M. Louarn, personal communication; T.M.H. and P.L.K., unpublished results), and in the plasmids R6K, R1, and R100 (6, 7).In addition to the terminator signals, a trans-acting gene called tus (termination utilization substance) has been identified that maps near 72 and that is required for function ofthe terminator signal Ti (5). In this paper, we present further characterization of tus. Complementation of tus mutants was used to locate the tus gene and the nucleotide sequence of the tus coding region was determined,* identifying an open reading frame (ORF) which encodes the tus gene product. We demonstrate the physical location of the tus gene in relationship to the terminator signal 72 and show that tus is also required for 72 function. Finally, we show that the tus gene encodes a DNA-binding protein which associates with the terminator signal sequences.

The role of chemokine–matrix interactions in integrin-dependent T-cell migration was examined to address the critical question of how chemokines provide directional information. The chemokine SDF-1α binds fibronectin (Fn) with a low nanomolar Kd(equilibrium dissociation constant). SDF-1α presented by Fn induced directed migration. Spatial concentration gradients of chemokine were not required to maintain directed migration. Fn-presented chemokine induced the polarization of cells, including the redistribution of the SDF-1α receptor, to the basal surface and leading edge of the cell. A new model for directed migration is proposed in which the co-presentation of an adhesive matrix and chemokine provides the necessary positional information independent of a soluble spatial gradient.