Involvement of Elongation Factor-1α in Cytokinesis without Actomyosin Contractile Ring in the Primitive Red Alga Cyanidioschyzon merolae

Cytokinesis is a pivotal event in cell division in all living organisms. In mammals, cytokinesis progresses via constriction of the contractile ring, a bundle of actin filaments and myosin that is located on the equatorial plane. The eukaryotic elongation factor 1 alpha (eEF-1 alpha) is associated with the contractile ring in sea urchin eggs. Although eEF-1 alpha is a ubiquitous protein translation factor and is highly conserved in eukaryotes, archaea, and prokaryotes, the archaebacterial EF-1 alpha and the prokaryotic EF-1 alpha ortholog EF-Tu do not function in cytokinesis, suggesting that the association between the contractile ring and EF-1 alpha appeared at about the same time as the establishment of eukaryotic cells. However, the role of EF-1 alpha in cytokinesis in primitive cells is unclear. In this study, we show that the primitive alga Cyanidioschyzon merolae elongation factor-1 alpha (CmEF-1 alpha) is localized in the contractile division plane, but not in the actomyosin contractile ring. The genome of C. merolae contains 2 unexpressed actin genes and lacks a myosin gene. Immunoblotting analyses revealed that the protein level of CmEF-1 alpha remained constant during the G1 to M phase, and then peaked during cytokinesis. Immunofluorescent microscopy revealed 2 patterns of localization of CmEF-1 alpha during the cell cycle: it was dispersed throughout the cytoplasm from G1 to early M phase, and then localized to the contractile region during cytokinesis. From the results of current study on cytokinesis in the primitive red algae C. merolae, our findings led us to hypothesize that when first established in the lower eukaryotes, cytokinesis fundamentally relied on eEF-1 alpha, and the actomyosin system was acquired later during evolution.