Cell‐cycle modulation of MPM‐2‐specific spindle pole body phosphorylation in Aspergillus nidulans

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Abstract
MPM‐2 is a monoclonal antibody that interacts with mitosis‐specific phosphorylated proteins in many different organisms. Immunocytochemistry of tissue culture cells has shown that MPM‐2 stains centrosomes, chromosomes, kinetochores, and spindles. In this paper, we demonstrate that MPM‐2 staining colocalizes with the spindle pole body (SPB) of Aspergillus nidulans and that SPB staining varies during the mitotic cycle. In an unsynchronized population, about one‐fourth to one‐third of the cells stain with MPM‐2 at the spindle plaques or SPBs. Nuclei in mitosis have two SPBs localized at the ends of the spindle, both of which stain with MPM‐2. To determine when MPM‐2 staining appears, we have examined the effects of temperature‐sensitive cell‐cycle mutations that block nuclear division in S or G2. Only a very small fraction of cells blocked in S‐phase stain with MPM‐2. In contrast, a large fraction of cells blocked in G2 stain brightly at the SPB. These data suggest that MPM‐2 reactivity of SPBs appears in G2. Moreover, the fact that cells blocked in G2 showed MPM‐2 staining but no spindles suggests that reactivity of SPBs occurs prior to mitosis but is not sufficient to trigger spindle formation. When G2‐blocked cells were downshifted to permissive temperature, they generated a mitotic spindle with an SPB at each end. Both SPBs stained with MPM‐2 in all of the mitotic cells.

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