Cell adhesion to extracellular matrix (ECM) proteins can generate transmembrane signals important for cell survival and can promote directed cell migration events. In a variety of cell types, integrin stimulation by ECM proteins such as fibronectin (FN) leads to changes in intracellular protein tyrosine phosphorylation events. In fibroblasts, the focal adhesion kinase (FAK), a nonreceptor protein-tyrosine kinase (PTK), colocalizes with integrin receptors at sites of cell attachment to ECM proteins. FAK may associate directly with ␤ integrin cytoplasmic domains (44) or may cocluster with integrin receptors through interactions with other integrin-associated proteins (4,8,22). FAK tyrosine phosphorylation is stimulated by cell binding to ECM proteins (for a review, see reference 50), by overexpression of the ␤ integrin cytoplasmic domains (52) and also by other growth factor or serum mitogens (for a review, see reference 24). Since integrin receptors lack catalytic activity, FAK association and activation may be important for integrin-mediated signal transduction events (for a review, see reference 20). This hypothesis is supported by gene knockout results in which both the FN-and FAK-null mice die as a result of similar developmental gastrulation defects (15,25).In addition to integrin stimulation of FAK, ECM protein binding to cells can lead to changes in the tyrosine phosphorylation of a number of different signaling proteins, including p130Cas , Shc, and Cbl, as well as structural proteins such as paxillin and tensin. Integrin stimulation can also promote increases in intracellular calcium levels (51), protein kinase C activity (32, 56), and phosphatidylinositol (PI) 3-kinase activity (7, 28). One downstream target for integrin-initiated signaling events is the activation of the extracellular signal-regulated kinase 2/mitogen-activated protein (ERK2/MAP) kinase pathway (9,38,39,47,59). Although integrin-initiated signaling to ERK2 is dependent on the integrity of the actin cytoskeleton and involves the activation of both the Rho and the Ras families of small GTPase proteins (12,40), the integrin signaling pathways upstream of Ras have not been clearly defined.Attempts to delineate the molecular mechanisms of integrin-stimulated signaling to ERK2 have yielded potentially conflicting results. In NIH 3T3 fibroblasts, Grb2 transiently binds to a motif surrounding FAK Tyr-925 after FN stimula-* Corresponding author. Mailing address:

A successful immune response depends on the capacity of immune cells to travel from one location in the body to another–these cells are rapid migrators, travelling at speeds of μm/minute. Their ability to penetrate into tissues and to make contacts with other cells depends chiefly on the β2 integrin known as LFA-1. For this reason, we describe the control of its activity in some detail. For the non-immunologist, the fine details of an immune response often seem difficult to fathom. However, the behaviour of immune cells, known as leukocytes (Box 1), is subject to the same biological rules as many other cell types, and this holds true particularly for the functioning of the integrins on these cells. In this Commentary, we highlight, from a cell-biology point of view, the integrin-mediated immune-cell migration and cell-cell interactions that occur during the course of an immune response.

A successful immune response depends on the migration of lymphocytes into lymph nodes or inflamed tissues where they make contact with antigen-presenting cells. We are interested in how one member of the integrin family, leukocyte function-associated antigen-1 (LFA-1), controls the function and, in particular, the migration of immune cells. We find that this integrin operates not only as an adhesion receptor for T lymphoblasts (T cells) but also induces their migration in vitro at approximately 15 microm/min. Migration requires active myosin light chain kinase at the leading edge and Rho kinase at the trailing edge of the cell. Two active conformations of LFA-1 are differently distributed on the T-cell membrane and regulate independent aspects of migration. High-affinity LFA-1 is located in a midcell 'focal zone' and influences the speed of migration, whereas intermediate affinity LFA-1 controls leading edge adhesions. Manipulating LFA-1 conformation in vivo can be performed, for example, by creating the active conformation in a transgenic mouse, and this model gives further insight into the role of LFA-1 in migration. In humans, the beneficial effect of functioning CD18 integrins in combating infections in vivo is illustrated by rare patients displaying two forms of leukocyte adhesion deficiency. In summary, we speculate that T cells have evolved a mode of rapid migration that is of paramount importance in achieving the high-speed immune surveillance upon which depends the body's protection against diverse invaders from pathogens to cancer cells.

Focal adhesion kinase–null (FAK−/−) fibroblasts exhibit morphological and motility defects that are reversed by focal adhesion kinase (FAK) reexpression. The FAK-related kinase, proline-rich tyrosine kinase 2 (Pyk2), is expressed in FAK−/− cells, yet it exhibits a perinuclear distribution and does not functionally substitute for FAK. Chimeric Pyk2/FAK proteins were created and expressed in FAK−/− cells to determine the impact of Pyk2 localization to focal contacts. Whereas an FAK/Pyk2 COOH-terminal (CT) domain chimera was perinuclear distributed, stable expression of a Pyk2 chimera with the FAK-CT domain (Pyk2/FAK-CT) localized to focal contact sites and enhanced fibronectin (FN)-stimulated haptotactic cell migration equal to FAK-reconstituted cells. Disruption of paxillin binding to the FAK-CT domain (S-1034) inhibited Pyk2/FAK-CT localization to focal contacts and its capacity to promote cell motility. Paxillin binding to the FAK-CT was necessary but not sufficient to mediate the indirect association of FAK or Pyk2/FAK-CT with a β1-integrin–containing complex. Both FAK and Pyk2/FAK-CT but not Pyk2/FAK-CT S-1034 reconstituted FAK−/− cells, exhibit elevated FN-stimulated extracellular signal–regulated kinase 2 (ERK2) and c-Jun NH2-terminal kinase (JNK) kinase activation. FN-stimulated FAK or Pyk2/FAK-CT activation enhanced both the extent and duration of FN-stimulated ERK2 activity which was necessary for cell motility. Transient overexpression of the FAK-CT but not FAK-CT S-1034 domain inhibited both FN-stimulated ERK2 and JNK activation as well as FN-stimulated motility of Pyk2/FAK-CT reconstituted cells. These gain-of-function studies show that the NH2-terminal and kinase domains of Pyk2 can functionally substitute for FAK in promoting FN-stimulated signaling and motility events when localized to β-integrin–containing focal contact sites via interactions mediated by the FAK-CT domain.