PITPβ does not have transfer activity but has cofactor activity. (A) In vitro [³H]-labeled PtdIns transfer activity from microsomes (permeabilized HL60 cells) to liposomes (PC:PI :: 98:2) is mediated by platelet PITPα, but not PITPβ. (B.C) Lipid kinase assays were performed to determine the effects of platelet PITPα (left) and PITPβ (right) on PtdInsP synthesis in vitro, before and after thrombin stimulation (3 minutes: time of thrombin stimulation [1 U/mL]). This assay, which does not require transfer activity, demonstrated that both PITP isoforms are required for phospholipid kinases to generate phosphoinositides. Phosphoinositide production in PITP-null platelets was restored by the addition of rPITPα and rPITPβ. ∗∗P < .01.