News and Publications

Integrins and Bidirectional Signaling

T.E. O'Toole, C. Buensuceso, D. Normand, L. Martin

Members of the integrin family of adhesion receptors are notable for their bidirectional signaling properties. In response to external agonists, cellular signals are transduced in an "inside out" fashion, inducing an extracellular conformational change of the integrin and allowing high-affinity ligand binding. After binding, events are transduced in an "outside in" fashion, inducing cellular responses such as growth, differentiation, survival, and migration. Previous work implicated the cytoplasmic domains of the and ß subunits of integrins in bidirectional signaling. Typical models propose that the interaction of these sequences with intracellular or membrane-bound proteins mediate these events.

To determine these mediators, we are using the yeast 2-hybrid approach. We recently discovered that the proteins Rack1 and eIF-2B bind to the tails of several integrins. Rack1 is a receptor for the ß isoform of activated protein kinase C, and eIF-2B is a regulatory subunit of a GTP-GDP exchange factor involved in protein translation. Currently, we are analyzing the functional significance of these associations. In particular, we wish to know if Rack1 mediates a physiologic association of the ß isoform or other isoforms of protein kinase C with integrins and if integrin signals through eIF-2B affect protein translation or other G protein exchange factors. To supplement the 2-hybrid approach in determining integrin-binding proteins, we are using adaptations of phage display and biopanning methods.

In a final area of investigation, we are examining the role of LIM domains in cell signaling. LIM domains are protein motifs characterized by a repetitive spacing of histidine and cysteine residues. They can coordinate zinc ions to form a 2-looped finger structure and are implicated in protein-protein interactions. Analysis of other binding partners of the LIM domain suggests that integrin tails have a primary structure necessary for recognition. Also, a number of proteins that contain LIM domains are found in focal contacts, cellular clusters of integrins, and other signaling molecules. Thus, we are examining the binding partners and functional properties of several LIM proteins potentially involved in integrin signaling.

PUBLICATIONS

Felding-Habermann, B., Silletti, S., Mei, F., Siu, C.-H., Yip, P., Brooks, P.C., Cheresh, D.A., O'Toole, T.E., Ginsberg, M.H., Montgomery, A.M.P. A single immunoglobulin-like domain of the human neural cell adhesion molecule L1 supports adhesion by multiple vascular and platelet integrins. J. Cell Biol. 139:1567, 1997.