Localization of spectrin isoforms at high resolution. (A-E) Localization of erythroid and nonerythroid spectrin isoforms in ultrathin sections of mouse MKs. Immunogold labeling of sections was performed with anti–β2 spectrin (A,B), anti–α2 spectrin (C), anti–α1 spectrin (D), and anti–β1 spectrin (E) antibodies. Gold particles (10-nm) recognizing anti–β2 spectrin (A-B) are evident on the plasma membrane and invaginated membranes of MKs. Gold particles recognizing anti–α2 spectrin (C) are also found on MK membranes. Gold particles recognizing anti–α1 spectrin (D) and anti–β1 spectrin (E) stained multivesicular bodies of MKs. Scale bar represents 200 nm. (F-I) Localization of spectrin isoforms in the detergent-insoluble cytoskeletons of proplatelets and platelets. Immunoelectron microscopic studies were used to localize individual spectrin isoforms in the membrane skeletons of proplatelets (F-G) and platelets (H-I). Preparations were incubated with affinity-purified anti-β2 (F,H) and anti–β1 spectrin (G,I) antibodies, followed by 10-nm gold particles coated with secondary antibodies. Scale bar shown in panel I indicates 100 nm. Proplatelets labeled with anti–β2 spectrin (F) and anti–β1 spectrin (G) have similar staining patterns, labeling the strands composing the membrane skeleton. Human platelet cytoskeletons were labeled with anti–β2 spectrin (H) and anti–β1 spectrin (I). β2 spectrin gold labeling was increased, whereas β1 was decreased in the platelet cytoskeleton. The gold particle density per square micrometer of cytoskeleton preparation was 53 ± 10 for anti–β2 spectrin of proplatelets (F), 69 ± 8 for anti–β1 spectrin of proplatelets (G) 232 ± 25 for anti–β2 spectrin of platelets (H), and 4 ± 2 for anti–β1 spectrin of platelets. Data are provided as means ± SE (n = 20). Experiments were carried out in triplicate. Scale bar represents 100 nm.