Fractioned distribution of PITPα and PITPβ within the platelets. (A) Representative immunoblot and (B) densitometry quantification of wild-type platelets indicates that PITPα is mostly distributed in the cytosol, whereas PITPβ has a disproportionate amount of total protein in the membrane and cytoskeleton (n = 3 separate experiments) in both resting and thrombin-activated platelets (1 U/mL for 1 minute). (C) Representative immunoblot of the fractioned distribution of PITPα in platelets lacking PITPβ. (D) Densitometry quantification of PITPα fractioned distribution in platelets lacking PITPβ. (E) Distribution of PITPβ in platelets lacking PITPα (bottom). (F) Fractioned distribution of PITPα and PITPβ in resting (left) and thrombin-activated (right) human platelets. (G) Densitometry quantification of PITPα and PITPβ in different cellular fractions of resting human platelets. All densitometry data from 3 separate experiments were summed and the percentage relative to the total single PITP isoform was plotted for each fraction. (H) In this model, PITPα serves the traditional role of transferring PtdIns from 1 subcellular compartment to another, such as the plasma membrane, and PITPβ, in turn, serves as a cofactor for PI kinase–mediated PIP synthesis.