9 1985 by The Humana Press Inc. All rights of any nature whatsoever reserved. 0163-4992/85/0700-0197502.00 Effect of Preferential Insertion of LDL Receptors Near Coated Pits CARLA WOFSY AND HECTOR ECHAVARR[A-HERAS Department of Mathematics and Statistics, University of New Nlexico, Albuquerque, New Mexico 87131 AND BYRON GOLDSTEIN* Theoretical Division, Los A/amos National Laboratory, University of California, Los A/amos, New Mexico 87.545 Received February 4, 1985; Accepted March 22, 1985 ABSTRACT Recent experiments suggest that low density lipoprotein (LDL) receptors on human fibroblasts are not inserted into the plasma mem- brane uniformly, as earlier experiments indicated, but are inserted into specialized regions, called plaques, where coated pits form. If the consequent reduction in the time required for LDL receptors to dif- fuse to coated pits were significant, this could alter conclusions drawn from previous calculations based on the assumption that LDL receptors are inserted uniformly. In particular, the conclusion could be wrong that diffusion of LDL receptors to coated pits is the rate lim- iting step in the interaction of cell surface LDL receptors with coated pits. Here we calculate the extent of the reduction in mean travel time of an LDL receptor to a coated pit, as a function of the plaque radius. We find that only if LDL receptor insertion is limited to a very small portion of the plasma membrane near coated pit sites is there a sub- stantial decrease in the average time it would take an LDL receptor to diffuse to a coated pit. In order for preferential insertion of LDL re- ceptors into plaques to cut the mean receptor travel time in half, plaques would have to take up no more than 10% of the cell surface area; to reduce the travel time by a factor of 10. plaques would have to *Author to whom all correspondence and reprint requests should be addressed. Cell Biophysics 197 Vol. Z, 1985