Patterns of tubulin isotype synthesis and usage during mitotic spindle morphogenesis in Physarum

Tubulin synthesis in the naturally synchronous plasmodium of Physarum polycephalum is a markedly periodic event restricted to the late G2 period of the cell cycle. Mitosis in the plasmodium is intranuclear, and there are no cytoplasmic microtubules at any stage of the cell cycle. We have combined a biochemical investigation of the synthesis of the plasmodial tubulin isotypes and their participation in the mitotic spindle with a microscopic study (immunofluorescence) of the development of spindle microtubules throughout the cell cycle. We have shown that all four tubulin isotypes identified in the plasmodium (α1, α2, β1 and β2) are present in the mitotic spindle. The stoichiometry of isotype usage in the mitotic spindle generally reflects the overall abundance of isotypes in the plasmodium as a whole: β2 > α1 > α2 > β1. We have also shown that tubulins synthesized in the G2 period of one cell cycle can be incorporated into the spindles of the immediately ensuing mitosis and have sufficient biological longevity to allow participation in the mitotic divisions of future cell cycles. Thus, the phenomenon of periodic tubulin synthesis does not reflect a restricted use of tubulin to the cell cycle in which it was synthesized. The major polymerization of tubulin in the nucleus occurred less than 30 min before metaphase. A novel tubulin‐containing structure was, however, present in the nucleus approximately 60 min before metaphase. Polymerized tubulin is rapidly removed from the nucleus following nucleokinesis.