The integrin alpha-subunit leg extends at a Ca2+-dependent epitope in the thigh/genu interface upon activation.
Two activation-dependent Abs to the integrin alphaL-subunit were used to study conformational rearrangement of alphaLbeta2 on the cell surface. Activation lowered the concentration of Ca2+ required for maximal expression of each epitope. Each Ab requires the Ca2+-binding loop in the integrin genu and nearby species-specific residues in the thigh domain. Key thigh residues are shielded from Ab in the bent integrin conformation by the alpha-subunit calf-1 domain and the nearby bent beta leg, suggesting that extension at the genu is required for epitope exposure. Activating stimuli and alpha/beta I-like small molecule antagonists demonstrate that exposure of epitopes in the integrin alpha- and beta-subunit legs is coordinate during integrin activation. A coordinating residue donated by the calf-1 domain is as important as Ca2+ for mAb binding. Together with inspection of the alphaV structure, this result suggests that the genu/calf-1 interface is maintained in integrin activation, and that extension occurs by a rearrangement at the thigh/genu interface.
Xie
Can
C
CBR Institute for Biomedical Research and Departments of Pathology and Anesthesia, Harvard Medical School, 200 Longwood Avenue, Boston, MA 02115, USA.
Shimaoka
Motomu
M
Xiao
Tsan
T
Schwab
Pascale
P
Klickstein
Lloyd B
LB
Springer
Timothy A
TA
D000595:Amino Acid Sequence; D002118:Calcium / Q000502:physiology*; D002460:Cell Line; D018604:Epitope Mapping; D000939:Epitopes / Q000276:immunology / Q000502:physiology*; D005434:Flow Cytometry; D005455:Fluorescent Antibody Technique; D006801:Humans; D039001:Integrin alpha Chains / Q000737:chemistry / Q000502:physiology*; D008969:Molecular Sequence Data; D017386:Sequence Homology, Amino Acid
D000939:Epitopes; D039001:Integrin alpha Chains; D002118:Calcium