Expression of β3 integrins with mutant cytoplasmic tails in β3-deficient mouse platelets. (A) The amino acid sequence of the cytoplasmic tail of the mouse β3 integrin subunit and the sites of alanine substitution for mutant subunits (red letters) are shown. The predicted sites of talin and src-family kinase binding are indicated by lines. (B) Retroviral expression of GFP and mutant β3 integrins in the platelets of reconstituted animals. Surface β3 integrin expression and GFP expression were measured in live platelets using flow cytometry. Platelets were harvested from animals reconstituted with β3-deficient fetal liver exposed to retroviral vectors encoding GFP only (vector), wild-type β3 (β3WT), and the β3 mutants indicated. The dot blots shown are representative of 7 animals of each type studied. Note the generally close correlation between GFP and β3 levels. (C) Expression of mutant β3 integrins on the platelet surface. Shown are histograms of surface β3 integrins in GFP-positive platelets from animals reconstituted with β3-deficient hematopoietic cells expressing the indicated mutant β3 subunits.