ROLE OF ACTIN-BINDING PROTEIN IN INSERTION OF ADHESION RECEPTORS INTOTHE MEMBRANE

The goal of this study was to determine whether actin-binding protein (ABP) regulates membrane composition, ABP-deficient and ABP-containing cells were transfected with the cDNAs coding for glycoprotein (GP) Ib -IX, a platelet receptor that interacts with ABP, Most of the GP Ib-IX remained inside the ABP-deficient cells, When ABP was present, functi onal GP Ib-IX was inserted into the membrane, GP Ib-IX lacking the dom ain that interacts with ABP also showed increased membrane insertion i n ABP-expressing cells. Furthermore, a fragment of ABP that lacks the dimerization and GP Ib-IX-binding sites restored the spreading of the cells and increased the amount of GP Ib-IX in the membrane, Finally, e xpression of ABP also increased endogenous beta(1) integrin in the mem brane, These results indicate that 1) ABP maintains the properties of the cell such that adhesion receptors can be efficiently expressed in the membrane; 2) increased receptor expression is accompanied by incre ased ability of the cell to spread; and 3) ABP exerts its effect by a mechanism that does not appear to involve direct cross-linking of acti n filaments or direct interaction with receptors.