ELLIPTIC VERSUS CIRCULAR ERYTHROCYTE MARGINAL BANDS - ISOLATION, SHAPE CONVERSION, AND MECHANICAL-PROPERTIES

Differentiation of nucleated erythrocytes involves transformation from spheroids to flattened discoids to mature flattened ellipsoids. The m arginal band (MB) of microtubules is required for this process and con tinues to play a role in maintaining mature ellipsoidal cell shape. On e hypothesis for MB function is that cell ellipticity is generated and maintained by asymmetric application of force across a flexible, circ ular MB frame by the membrane skeleton or other transverse elements. T his is based on an earlier finding that isolated erythrocyte MBs are m uch more circular than MBs in situ. However, our present studies of sa lamander erythrocyte MBs isolated by a detergent-based method challeng e this hypothesis. Most of these isolated MBs are initially elliptical , even though they lack transverse material (= E-MBs). They can be sta bilized in that form for long periods and can be convened experimental ly into the circular form (= C-MBs) by extended incubation in isolatio n medium or by treatment with elastase or subtilisin. We have tested a n alternative hypothesis for generation and maintenance of ellipsoidal MBs, one based on intrinsic differential bending resistance and suppo rted by construction of models. Using laser microsurgical transection to compare mechanical responses of isolated E-MBs and C-MBs, we have f ound their behavior to be guile different. Whereas C-MBs linearize, mo st E-MBs do not, instead retaining considerable curvature. These resul ts are incompatible with the differential bending resistance hypothesi s, which predicts both C-MB and E-MB linearization, However, they are consistent with a third model, in which material bound to the MB stabi lizes it in the mature ellipsoidal form, and indicate that the mechani sm for maintenance of MB ellipticity differs from that involved in its generation. Cell Motil. Cytoskeleton 40:238-248, 1998. (C) Wiley-Liss , Inc.