Chimeras of the integrin beta subunit mid-region reveal regions required for heterodimer formation and for activation

Permissive transmembrane helix heterodimerization is required for the expression of a functional integrin

The current paradigm is that integrin is activated via inside-out signalling when its cytoplasmic tails and TMs (transmembrane helices) are separated by specific cytosolic protein(s). Perturbations of the helical interface between the α- and β-TMs of an integrin, as a result of mutations, affect its function. Previous studies have shown the requirement for specific pairing between integrin subunits by ectodomain-exchange analyses. It remains unknown whether permissive α/β-TM pairing of an integrin is also required for pairing specificity and the expression of a functionally regulated receptor. We performed scanning replacement of integrin β2-TM with a TM of other integrin β-subunits. With the exception of β4 substitution, others presented β2-integrins with modified phenotypes, either in their expression or ligand-binding properties. Subsequently, we adopted αLβ2 for follow-on experiments because its conformation and affinity-state transitions have been well defined as compared with other members of the β2-integrins. Replacement of β2- with β3-TM generated a chimaeric αLβ2 of an intermediate affinity that adhered to ICAM-1 (intercellular adhesion molecule 1) but not to ICAM-3 constitutively. Replacing αL-TM with αIIb-TM, forming a natural αIIb/β3-TM pair, reversed the phenotype of the chimaera to that of wild-type αLβ2. Interestingly, the replacement of αLβ2- with β3-TM showed neither an extended conformation nor the separation of its cytoplasmic tails, which are well-reported hallmarks of an activated αLβ2, as determined by reporter mAb (monoclonal antibody) KIM127 reactivity and FRET (fluorescence resonance energy transfer) measurements respectively. Collectively, our results suggest that TM pairing specificity is required for the expression of a functionally regulated integrin.

Down-regulation of integrin alpha M beta 2 ligand-binding function by the urokinase-type plasminogen activator receptor

The cell adhesion molecule integrin alphaMbeta2 associates with the urokinase-type plasminogen activator receptor (uPAR) on monocytes and neutrophils. uPAR also associates with members of the beta1 and beta3 integrins, and it modulates the ligand-binding function of these integrins. In this study, we showed that co-expressing uPAR with alphaMbeta2 in 293 transfectants down-regulated the ligand-binding capacity of alphaMbeta2 to denatured protein, fibrinogen, and intercellular adhesion molecule 1 (ICAM-1). Migration of transfectants on fibrinogen mediated by alphaMbeta2 was reduced in the presence of uPAR. In addition, the constitutive ligand-binding property of an alphaMbeta2 mutant was attenuated by its association with uPAR. Co-immunoprecipitation analyses using a panel of alphaMbeta2-specific mAbs suggest shielding of the ligand-recognition site of alphaMbeta2 by uPAR.

Integrin structure: new twists and turns in dynamic cell adhesion

The divalent-cation-dependent binding of alphabeta heterodimeric integrins to their ligands regulates most cellular processes. Integrin-ligand interactions are tightly controlled by inside-out activation signals. Ligand-bound integrins in turn transduce outside-in signals typical of other receptors. Precise information of how ligands bind to integrins is restricted to that of a small vWF A-type domain present in some alpha-subunits (alphaA). Both inside-out and outside-in signals elicit tertiary and quaternary changes in integrins, but the precise nature and scope and of these changes are unknown. The recently solved structures of the extracellular segment of integrin alphaVbeta3 in its unliganded and liganded states are generating exciting new insights into the design, wiring, function and regulation of this protein family. The structures reveal a surprising degree of flexibility at defined regions in the structure that is potentially controlled by cations. The quaternary structure of the ligand-binding region bears a striking resemblance to the nucleotide-binding pocket of G-proteins, implying analogous activation and signaling mechanisms. Structural links exist through which ligand-induced tertiary changes may be translated into quaternary changes and vice versa. The structures also raise the tantalizing hypothesis that alphaA is a regulated endogenous integrin ligand, so that no special regulatory features are needed in this integrin. These findings provide the framework for new investigations of structure-activity relationships in integrins, with important implications for targeting these receptors therapeutically [corrected].