Colocalization of Dynactin Subunits P150Glued and P50 with Melanosomes in Normal Human Melanocytes

Melanocytic dendrites consist of a central core of microtubules (MT) and a subcortical actin network. In previous reports we showed the presence of MT‐associated motor proteins kinesin and cytoplasmic dynein on the melanosomal surface, forming a link with MT (Vancoillie et al. J Invest Dermatol 2000;114:421–429; Vancoillie et al. Br J Dermatol 2000;143:258–306). We could also demonstrate the association of kinectin, the kinesin receptor, with melanosomes. The interaction of cytoplasmic dynein with its cargoes is thought to be indirectly mediated by dynactin, a complex that binds to the dynein intermediate chain. Therefore, in this study, we investigated the in vitro expression of dynactin subunits P150Glued and P50 in normal human epidermal melanocytes, keratinocytes, and dermal fibroblasts by reverse transcription‐polymerase chain reaction and northern blot analysis. In an attempt to gain an insight into the subcellular localization of dynactin, immunofluorescence and immunoelectron microscopy (IEM) studies were performed. The two isoforms of P150Glued and P50 are expressed in all studied skin cells. Immunofluorescence staining shows punctate distributions for P150Glued and P50 in melanocytes. P150Glued shows a clear centrosomal staining and accentuation in the dendrite tips. P50 is also accentuated in the perinuclear area and dendrite tips. Immunofluorescence double‐labeling with a melanosome marker showed apparent colocalization of both P150Glued and P50 with melanosomes. By IEM, P50 is detected on the surface of the majority of melanosomes in melanocytes. The colocalization of different subunits of the dynactin complex with melanosomes is consistent with the earlier finding of cytoplasmic dynein association with melanosomes and supports the hypothesis that this complex could form a link between cytoplasmic dynein and the melanosomal membrane.