Surface Functions during Mitosis

The surface distribution of concanavalin A (Con A) bound to cell membrane receptors varies dramatically as a function of mitotic phase. The lectin is distributed diffusely on cells labeled and observed between mid-prophase and early anaphase, whereas cells observed in late anaphase or telophase demonstrate a marked accumulation of Con A-receptor complexes over the developing cleavage furrow (Berlin, Oliver, and Walter . 1978 . Cell . 15:327341) . In this report, we first use a system based on video intensification fluorescence microscopy to describe the simultaneous changes in cell shape and in lectin-receptor complex topography during progression of single cells through the mitotic cycle. The video analysis establishes that fluorescein succinyl Con A (F-S Con A)-receptor complex redistribution begins coincident with the first appearance of the cleavage furrow and is essentially complete within 2-3 min. This remarkable redistribution of surface fluorescence occurs during only a modest change in cell shape from a sphere to a belted cylinder . It reflects the translocation of complexes and not the accumulation of excess labeled membrane in the cleavage furrow : first, bound fluorescent cholera toxin which faithfully outlines the plasma membrane is not accumulated in the cleavage furrow, and, second, electron microscopy of peroxidase-Con A labeled cells undergoing cleavage shows that there is a high linear density of lectin within the furrow while Con A is virtually eliminated from the poles. The rate of surface movement of F-S Con A was quantitated by photon counting during a repetitive series of laser-excited fluorescence scans across dividing cells . Results were analyzed in terms of two alternative models of movement : a flow model in which complexes moved unidirectionally at constant velocity, and a diffusion model in which complexes could diffuse freely but were trapped at the cleavage furrow . According to these models, the observed rates of accumulation were attainable at either . an effective flow velocity of ^-1 ,um/min, or an effective diffusion coefficient of -10-9 cm'/s. However, in separate experiments the lectin-receptor diffusion rate measured directly by the method of fluorescence recovery after photobleaching (FRAP) on metaphase cells was only _10-10 cm2/s . Most importantly, photobleaching experiments during the actual period of F-S Con A accumulation showed that lectin-receptor movement during cleavage occurs unidirectionally . These results rule out diffusion and make a process of oriented flow of ligand-receptor complexes the most likely mechanism for ligand-receptor accumulation in the cleavage furrow . The distribution of surface-bound concanavalin A (Con A) varies dramatically with mitotic phase, remaining diffuse from prophase through metaphase and then showing a striking accumulation in the cleavage furrow at anaphase and telophase (2). The movement of Con A-receptor complexes into the 950 cleavage furrow appears analogous to their movement into the pseudopods of phagocytic cells, the uropods of oriented cells, or into the protuberance of cells forming caps (1, 24). The array of hypotheses developed to explain the movement of lectin-receptor and antibody-receptor complexes (eg., referTHE JOURNAL OF CELL BIOLOGY " VOLUME 93 JUNE 1982 950-960 ©The Rockefeller University Press 0021-9525/82/06/0950/11 $1 .00 on A ril 9, 2017 D ow nladed fom Published June 1, 1982