The role of NuMA in the interphase nucleus.

NuMA is an essential protein for the formation of spindle poles in mitosis. During interphase, NuMA is transported into the nucleus where it resides until prometaphase of the next mitotic cycle. We tested for a potential function of NuMA in interphase nuclei that were assembled from human sperm DNA using frog egg extract immunodepleted of NuMA. Despite the absence of NuMA, nuclei formed without visible changes of the chromatin structure, surrounded by an intact nuclear membrane containing pores and nuclear lamins. These nuclei were fully competent to import nuclear substrates and to replicate their DNA. By screening tissue sections of various organs, absence of NuMA from the nucleus was observed in a number of cell types, including sperm, granulocytes in the blood, and differentiated smooth and skeletal muscle fibers. Experiments on cultured myoblasts indicated that NuMA is degraded during muscle cell differentiation. The absence of NuMA in interphase nuclei of the tissues tested correlated with a non-spherical, elongated or beaded nuclear morphology, suggesting that during interphase NuMA may act as a non-essential nucleoskeletal element.