Association of kinesin and myosin with pigment granules in crustacean chromatophores

Chromatic adaptation in crustaceans results from the differential distribution of colored pigment granules within their chromatophores consequent to cell signaling by neurosecretory peptides. However, the force transducing, mechanochemical protein motors responsible for granule translocation, and their molecular mechanisms of action, are not well understood. The present study uses immunocytochemical techniques and a motility assay in vitro to demonstrate that protein motors from the kinesin and myosin superfamilies are stably associated with membrane-bounded pigment granules in the red, ovarian chromatophores of the freshwater, palaemonid shrimp, Macrobrachium olfersii. Monoclonal antibodies against conventional kinesin heavy chain, and an anti-myosin whole serum, labeled pigment-containing fragments prepared from homogenates of chromatophores with fully dispersed or aggregated pigments: this finding infers a permanent association between the protein motors and the pigment granules, and suggests that such motors may be regulated while bound to their cargos. The pigment aggregator appears to be a myosin since the anti-myosin whole serum attenuated hormonally triggered pigment aggregation in the motility assay in vitro, and induced pigment hyper-dispersion in some chromatophores. Western blots of the chromatophore-containing, ovarian tissue homogenate demonstrated protein bands consistent with myosin II and myosin XII, either of which may be the pigment aggregator. This study provides the first direct evidence for myosin and kinesin protein motors directly and stably associated with pigment granules in crustacean chromatophores, and may represent the first successful isolation of myosin class XII.