Abstract
Previous work has shown that bovine prothrombin fragment 1 binds to supported planar membranes composed of phosphatidylcholine and phosphatidylserine in a Ca 2+-specific manner (Tendian et al. (1991) Biochemistry 30, 10991; Pearce et al. (1992) Biochemistry 31, 5983–5995). In the present work, fluorescence pattern photobleaching recovery has been used to examine the effect of membrane-bound fragment 1 on the translational diffusion coefficients of two fluorescent phospholipids in fluid-like phosphatidylserine/phosphatidylcholine Langmuir-Blodgett monolayers. The results show that saturating concentrations of fragment 1, in the presence of Ca 2+, reduce the diffusion coefficient of nitrobenzoxadiazolyl-conjugated phosphatidylserine (NBD-PS) and nitrobenzoxadiazolyl-conjugated phosphatidylcholine (NBD-PC) by factors of approximately four and two, respectively. Ca 2+ or fragment 1 alone do not have a statistically significant effect on NBD-PS or NBD-PC diffusion. In addition, a nonspecific protein (ovalbumin) does not change the diffusion coefficients of the fluorescent phospholipids either in the absence or presence of Ca 2+. The fractions of the fluorescent phospholipids that are laterally mobile are approximately 0.9 for all samples. These results are interpreted with several models for possible mechanisms by which extrinsically bound proteins might retard phospholipid diffusion in membranes.
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