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Identification of cell adhesive active sites in the N-terminal domain of thrombospondin-1.

ABSTRACT: Using a series of fusion proteins that span almost all of the thrombospondin-1 (TSP-1) molecule, we observed in this study that Chinese hamster ovary (CHO) K1 cells strongly attached to the N-terminus but not to the other domains of TSP-1 (e.g. the C-terminus, and type 1, type 2 and type 3 repeats). In addition, attachment to the N-terminus of CHO S745 cells defective in cell-surface glycosaminoglycans (GAGs) was decreased by 47% compared with that observed with CHO K1 cells, indicating the presence of GAG-dependent cell adhesive sites. With the aim of identifying these cell adhesive sites, a series of synthetic peptides, overlapping heparin-binding sequences ARKGSGRR (residues 22-29), MKKTRG (residues 79-84) and TRDLASIARLRIAKGVNDNF (residues 170-189), were synthesized and tested for their ability to support CHO cell attachment. Using both centrifugation and cell-attachment assays, MKKTRG-containing peptides promoted CHO K1 cell adhesion, while ARKGSGRR-containing peptides and peptide TRDLASIARLRIAKGVNDNF did not. CHO S745 cell attachment to MKKTRG-containing peptides was partially decreased. A 36% decrease in CHO K1 cell attachment to the N-terminus was also observed when the heparin-binding consensus sequence KKTR was mutated to QNTR. In addition, peptide MKKTRG partially inhibited (25% inhibition) CHO K1 cell attachment to the N-terminus. However, peptide MKKTRG was not sufficient to fully promote cell attachment to the N-terminus of TSP-1. Peptides VDAVRTEKGFLLLASLRQ and TLLALERKDHS also supported CHO K1 cell attachment in a GAG-dependent and -independent manner respectively. Moreover, CHO K1 cell attachment to MKKTRG was found to be markedly enhanced when flanked with the sequences VDAVRTEKGFLLLASLRQ and TLLALERKDHS. Peptide VDAVRTEKGFLLLASLRQMKKTRG nearly abolished (98% inhibition) CHO K1 cell attachment to the N-terminus, while peptides MKKTRG, MKKTRGTLLALERKDHS and VDAVRTEKGFLLLASLRQ had only a moderate inhibitory effect (25, 27 and 53% inhibition respectively). These data indicate that the sequence VDAVRTEKGFLLLASLRQMKKTRGTLLALERKDHS (residues 60-94) constitutes a GAG-dependent cell adhesive site in the N-terminus of TSP-1. Moreover, a GAG-independent site, encompassing residues 189-200 (FQGVLQNVRFVF), has been identified. These two adhesive sites supported the attachment of a wide variety of cells (human breast carcinoma, melanoma and osteosarcoma cells), and a high degree of sequence homology was found between TSP-1 and TSP-2 between residues 60 and 94 (48% identity) and 189-200 (67% identity), further suggesting the functional importance of these two cell adhesive sites in the N-terminus of TSP-1.

SUBMITTER: Clezardin P

PROVIDER: S-EPMC1218140 | biostudies-other | 1997 Feb

REPOSITORIES: biostudies-other

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Project description:HIV integrase (HIV-IN), one of three HIV enzymes, is a target for the treatment of AIDS, but the full biological assembly has been difficult to characterize, hampering inhibitor design. The recent crystallographic structures of integrase from prototype foamy virus (PFV-IN) with bound DNA were a breakthrough, revealing how viral DNA organizes two integrase dimers into a tetramer that has the two active sites appropriately spaced for insertion of the viral DNA into host DNA. The organization of domains within each PFV-IN protein chain, however, varies significantly from that found in HIV-IN structures. With the goal of identifying shared structural characteristics, the interactions among components of the PFV-IN and HIV-IN assemblies were investigated with the macromolecular docking program DOT. DOT performs an exhaustive, rigid-body search between two macromolecules. Computational docking reproduced the crystallographic interactions of the PFV-IN catalytic and N-terminal domains with viral DNA and found similar viral DNA interactions for HIV-IN. Computational docking did not reproduce the crystallographic interactions of the PFV-IN C-terminal domain (CTD). Instead, two symmetry-related positions were found for the PFV-IN CTD that indicate formation of a CTD dimer between the two active sites. Our predicted CTD dimer is consistent with cross-linking studies showing interactions of the CTD with viral DNA that appear to be blocked in the PFV-IN structures. The CTD dimer can insert two arginine-rich loops between the two bound vDNA molecules and the host DNA, a region that is unoccupied in the PFV-IN crystallographic structures. The positive potential from these two loops would alleviate the large negative potential created by the close proximity of two viral vDNA ends, helping to bring together the two active sites and assisting host DNA binding. This study demonstrates the ability of computational docking to evaluate complex crystallographic assemblies, identify interactions that are influenced by the crystal environment, and provide plausible alternatives.

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Identification of cell adhesive active sites in the N-terminal domain of thrombospondin-1.

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