The nuclear matrix is an integral part of nuclear structure which undergoes a profound reorganization during the cell cycle reflecting major changes in functional requirements. This includes the processes of DNA replication and gene expression at interphase and partitioning of the nuclear contents during mitosis. Using a monoclonal antibody (mAb2A) which specifically stains a novel nuclear meshwork which reorganizes during the cell cycle in Drosophila, we have initiated a study to: 1 ) more closely analyze this structural reorganization; 2) clone and characterize the antigens recognized by this antibody; and 3 ) isolate other interacting proteins in order to gain insight into the regulation of this process. The mAb2A-labeled structure changes from what appears as a diffuse meshwork at interphase to a distinct spindle-like scaffold at prophase. Since at metaphase the microtubules of the mitotic apparatus co-localize with the mAb2A spindle structure, a model is considered whereby the nuclear mAb2A-labeled scaffolding reorganizes during the cell cycle to provide a guide for the establishment of the mitotic apparatus. The mAb2A has identified two separate antigens, each of which shows similar distribution patterns. One of these antigens has been partially cloned and contains an unusual tandem ser-thr kinase domain. The association of this kinase homologue with a nuclear scaffold which reorganizes during the cell cycle suggests that it may be involved in regulating changes in nuclear architecture during the cell cycle andlor in mediating the downstream consequences of such changes. e 1996 Wiley-Liss, Inc.