EM negative-stain class averages of integrins αVβ3 and αXβ2 in bent and extended conformations (33, 43). The EM images of the extended conformations only are colored according to the scheme shown in d. (a) αVβ3 in bent ( panel 1), extended with closed headpiece ( panel 2), and extended with open headpiece ( panel 3) conformations. (b) αXβ2 in bent ( panel 1), extended with closed headpiece ( panel 2), and extended with open headpiece ( panel 3) conformations. (c) αXβ2 in complex with CBR LFA-1/2 Fab illustrates flexibility of the β leg: panel 1, closed headpiece with parallel legs; panel 2, closed headpiece with crossed legs; panels 3 and 4, open headpiece. Panels 1–3 are with clasped αXβ2, and panel 4 is with unclasped αXβ2. In a-c, a schematic in the same orientation as the right-most panel is shown to the right; the dashed lower β legs symbolize flexibility and averaging-out.