Domains in plexins: links to integrins and transcription factors

Integrins are adhesion molecules that bind diverse cell-surface and extracellular-matrix ligands 1 Hynes R.O. Cell. 1992; 69: 11-25 Abstract Full Text PDF PubMed Scopus (9503) Google Scholar . They are heterodimeric receptors, containing α and β subunits. No significant sequence similarity between the extracellular domains of integrin β subunits and any other protein has been reported, although the presence of a DXSXS motif, and secondary-structure predictions, suggests that the most-conserved region adopts an I-domain-like fold (also called a von Willebrand factor A domain) 2 Lee J-O. Rieu P. Arnaout M.A. Liddington R. Cell. 1995; 80: 631-638 Abstract Full Text PDF PubMed Scopus (816) Google Scholar , 3 Tozer E.C. et al. J. Biol. Chem. 1996; 271: 21978-21984 Crossref PubMed Scopus (161) Google Scholar , 4 Tuckwell D.S. Humphries M.J. FEBS Lett. 1997; 400: 297-303 Abstract Full Text Full Text PDF PubMed Scopus (101) Google Scholar . Furthermore, the C-terminal third of the extracellular domain contains four internal, cysteine-rich repeats 5 Tamkun J.W. et al. Cell. 1986; 46: 271-282 Abstract Full Text PDF PubMed Scopus (581) Google Scholar . The only other cysteine-rich region (a cluster of seven cysteine residues in 50 residues) is in the N-terminal segment of mature integrin β subunits. Six of these cysteines form disulfide bonds to one another; the remaining, first, cysteine forms a long-range disulfide bond that links the N-terminal and C-terminal cysteine-rich regions 6 Calvete J.J. Henschen A. González-Rodrı́guez J. Biochem. J. 1991; 274: 63-71 Crossref PubMed Scopus (170) Google Scholar .