Lateral diffusion of lectin receptors in fibroblast membranes as a function of cell shape

Anchorage-dependent fibroblasts respond to biochemical growth signals only when attached to and spread on a suitable substrate surface. Attachment of fibroblasts initiates a cytoskeletal assembly process that results in the organization of long actin stress fibers and microtubules which may be required for transmembrane signal transduction. Fibroblasts maintained in suspension, however, remain spherical with no apparent stress fibers or lengthy microtubules. Because of the significant differences in cytoskeletal organisation induced by shape modification, and the resulting possible changes in organization and dynamics of membrane receptors, the technique of fluorescence redistribution after photobleaching (FRAP) was employed to examine the lateral mobility of wheat germ agglutinin (WGA) and succinylated concanavalin A (sCon A) receptors in the plasma membrane of untransformed and Kirsten murine sarcoma virus-transformed Balb/c 3T3 fibroblasts in the spread and spherical state. An examination of FITC-WGA and FITC-sCon A binding to the plasma membrane for both cell lines in a spread or spherical state demonstrated no significant differences in the number of WGA or Con A receptors as a function of shape or transformation. The primary observations from this study are (a) membrane WGA and sCon A receptors in spherical Balb/c 3T3 fibroblasts display mobility 12 times faster than in the spread state, while phospholipid mobility is similar and apparently shape independent, (b) transformed cells in the spread state have WGA and sCon A receptor mobilities similar to those of untransformed cells in the spread state, (c) flat adherent, but not unattached spherical, Balb/c 3T3 fibroblasts are subject to Con A-induced global modulation and (d) transformed cells in the spherical state contain a significant population of cells (approximately 30%) with WGA receptor mobilities faster than those observed in spherical untransformed cells. These observations are discussed in terms of a linked matrix model for membrane protein diffusion.