A high molecular weight centrosomal protein of mammalian cells is antigenically related to myosin II

Available data on the molecular composition of the centrosome, the typical microtubule-organizing center of animal cells, are still fragmentary. To address this important issue we have taken advantage of centrosome isolation from a human lymphoblastic cell line (KE37) to generate a monoclonal antibody (mAb) library. Here we present the characterization of one of these mAbs (CTR56). On the basis of both its immunofluorescence staining pattern and its reactivity with a major 200 kD antigen on immunoblots, CTR56 has been tentatively classified as an anticellular myosin heavy chain. In light of cytological and biochemical data obtained in parallel with two other well-characterized myosin antibodies, it appears that myosin cannot be considered as a genuine centrosomal protein. We have resolved the paradoxical results with CTR56 by showing that in addition to the cellular myosin heavy chain, this antibody also recognizes a high molecular weight protein specifically enriched in centrosomal fractions. The possible biological significance of this finding is discussed in structural and functional terms.