Self-centering in cytoplasmic fragments of melanophores.

Abstract

The radial array of cytoplasmic microtubules (MTs) provides routes for intracellular transport and defines spatial organization of cytoplasm through interaction with molecular motors bound to membrane organelles (Kellogg et al., 1994 ; Hirokawa, 1998 ). The array is believed to be organized by the centrosome, which is capable of nucleating MTs. Our recent studies of pigment transport (Rodionov and Borisy, 1997a ) (reviewed in Haimo, 1997 ) and MT dynamics (Rodionov and Borisy, 1997b ) in cytoplasmic fragments of melanophores demonstrated that nucleation by the centrosome is not an exclusive pathway for organizing microtubules. We demonstrated that a radial array of cytoplasmic MTs can form by self-organization and, moreover, that a mechanism exists that maintains the focus of the array at the cell centroid. Fish melanophores are pigment cells whose only function is aggregation of pigment granules at the center or redispersion throughout the cytoplasm. The granules move along radial microtubules (MTs) by means of molecular motors of dynein (aggregation) or kinesin (dispersion) families (Schliwa, 1984 ; Obika, 1986 ; Haimo and Thaler, 1994 ). Microsurgically produced cytoplasmic fragments of melanophores organize a radial array of MTs with correct polarity orientation (plus ends at the periphery) and aggregate pigment at its center (Matthews, 1931 ; McNiven et al., 1984 ; McNiven and Porter, 1986 , 1988 ).