Diagrammatic illustrations of spatiotemporal regulation of integrin signaling by filamin. (A) At the resting state, integrin is stabilized by filamin that forms a ternary complex with both integrin α and β CTs.²⁹ Agonist stimulation triggers a cascade of cellular signals that lead to the recruitment/activation of talin to bind integrin. The activation of talin is mediated by PIP2 that is generated by PIPKIγ recruited by talin (not drawn; see the text). The talin-activating PIP2 also opens up the closed conformation of α CT (Figure 4D), which drastically enhances its binding to filamin accompanied by a major conformational change of the N-terminal of α CT from α-helix to β-strand. Note the dramatic change of the integrin cytoplasmic face where filamin initially links α/β CTs of inactive integrin with actin but is dissociated by talin (bound to β CT and actin) and then strongly reassociated with α CT of active integrin and actin to promote the outside-in signaling. The question mark indicates the complex actin cytoskeleton reassembly process as regulated by the spatial rearrangement of filamin. Note that in addition to filamin and talin drawn in this diagram, many molecules²³ including other integrin CT binding proteins⁶⁸ may be involved in mediating the outside-in signaling in a spatiotemporal manner, which remains to be investigated. Structures displayed in the diagram were modified from FLNa-Ig21/αIIb (7SC4) and protein data bank--integrin αVβ3 ectodomain (1JV2) and αIIbβ3 TMCT (2KNC), talin (2H7E), FLNa-Ig21/αIIb CT/β3 CT (2MTP). (B) Clustering of activated integrins as mediated by the dimerization of talin and filamin. Note that clustering may be also mediated by additional regulators, such as kindlin and paxillin.²⁴