Heterotrimeric kinesin II is the microtubule motor protein responsible forpigment dispersion in Xenopus melanophores

Melanophores move pigment organelles (melanosomes) from the cell center to the periphery and vice-versa. These bidirectional movements require cytopla smic microtubules and microfilaments and depend on the function of microtub ule motors and a myosin. Earlier we found that melanosomes purified from Xe nopus melanophores contain the plus end microtubule motor kinesin II, indic ating that it may be involved in dispersion (Rogers, S.L., I.S. Tint, P.C. Fanapour, and V.I. Gelfand. 1997. Proc. Natl. Acad. Sci, USA. 94: 3720-3725 ). Here, we generated a dominant-negative construct encoding green fluoresc ent protein fused to the stalk-tail region of Xenopus kinesin-like protein 3 (Xklp3), the 95-kD motor subunit of Xenopus kinesin II, and introduced it into melanophores. Overexpression of the fusion protein inhibited pigment dispersion but had no effect on aggregation. To control for the specificity of this effect, we studied the kinesin-dependent movement of lysosomes. Ne ither dispersion of lysosomes in acidic conditions nor their clustering und er alkaline conditions was affected by the mutant Xklp3. Furthermore, micro injection of melanophores with SUK4, a function-blocking kinesin antibody, inhibited dispersion of lysosomes but had no effect on melanosome transport . We conclude that melanosome dispersion is powered by kinesin II and not b y conventional kinesin. This paper demonstrates that kinesin II moves membr ane-bound organelles.