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Phospholipid transfer protein (PLTP) remodels high density lipoproteins (HDL) into large and small particles. It also mediates the dissociation of lipid-poor or lipid-free apolipoprotein A-I (apoA-I) from HDL. Remodeling is enhanced markedly in triglyceride (TG)-enriched HDL (Rye, K.-A., Jauhiainen, M., Barter, P. J., and Ehnholm. C. (1998) J. Lipid. Res. 39, 613-622). This study defines the mechanism of the remodeling of HDL by PLTP and determines why it is enhanced in TG-enriched HDL. Homogeneous populations of spherical reconstituted HDL (rHDL) containing apoA-I and either cholesteryl esters only (CE-rHDL; diameter 9.3 nm) or CE and TG in their core (TG-rHDL; diameter 9.5 nm) were used. After 24 h of incubation with PLTP, all of the TG-rHDL, but only a proportion of the CE-rHDL, were converted into large (11.3-nm diameter) and small (7.7-nm diameter) particles. Only small particles were formed during the first 6 h of incubation of CE-rHDL with PLTP. The large particles and dissociated apoA-I were apparent after 12 h. In the case of TG-rHDL, small particles appeared after 1 h of incubation, while dissociated apoA-I and large particles were apparent at 3 h. The composition of the large particles indicated that they were derived from a fusion product. Spectroscopic studies indicated that the apoA-I in TG-rHDL was less stable than the apoA-I in CE-rHDL. In conclusion, these results show that (i) PLTP mediates rHDL fusion, (ii) the fusion product rearranges by two independent processes into small and large particles, and (iii) the more rapid remodeling of TG-rHDL by PLTP may be due to the destabilization of apoA-I. Phospholipid transfer protein (PLTP)1 transfers phospholipids (PL) between high density lipoproteins (HDL) and very low density lipoproteins as well as between different particles within the HDL fraction (1, 2). It also remodels HDL into large and small particles in a process that is accompanied by the dissociation of lipid-poor or lipid-free apolipoprotein A-I (apoA-I) (3-8). Remodeling is enhanced markedly in HDL that contain triglyceride (TG) in their core (9). Evidence of the importance of PLTP in HDL metabolism comes from studies of mice transgenic for human PLTP. These animals have increased levels of pre-␤ 1 -migrating HDL, the initial acceptors of cellular cholesterol in the first step of the reverse cholesterol pathway, and are also resistant to intracellular cholesterol accumulation (10 -12). Studies of PLTP knockout mice have shown that PLTP is essential for maintaining normal HDL levels in plasma (13). Moreover, it has been reported recently that PLTP-mediated transfers of phospholipids between HDL and other lipoprotein classes are not interchangeable with the phospholipid transfers that are mediated by cholesteryl ester transfer protein (CETP) (14).The mechanism of the remodeling of HDL by PLTP is poorly understood. Although there is evidence that particle fusion and the dissociation of lipid-poor or lipid-free apoA-I are involved (7,8), nothing is known about how the interaction of PLTP wi...

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The cytoplasmic domain of the beta subunit of the alpha IIb beta 3 integrin is required for cell spreading on fibrinogen. Here we report that deletion of six amino acids from the COOH terminus of the beta 3 (I757TYRGT) totally abolished cell spreading and formation of adhesion plaques, whereas retaining Ile757 partially preserved these functions. We further found that substitution of Tyr747 with Ala also abolished alpha IIb beta a-mediated cell spreading. The effects of these and other mutations on additional functions of alpha IIb beta 3 were also studied. Progressive truncations of beta 3, in which stop codons were inserted at amino acid positions 759-756, caused partial defects in the recruitment of alpha IIb beta 3 to preestablished adhesion plaques and a gradual decrease in the ability of alpha IIb beta 3 to mediate internalization of fibrinogen-coated particles. The Tyr747-->Ala substitution mutant was almost totally inactive in both of these assays. Point mutations at Tyr759, and at a conserved area close to the transmembrane domain of beta 3, decreased integrin recruitment to preestablished adhesion plaques but allowed alpha IIb beta 3-mediated formation of these structures and partial cell spreading. Deletion of the cytoplasmic domain of beta 3 did not affect the constitutive endocytosis of alpha IIb beta 3.

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