MOST OF CENTRIN IN ANIMAL-CELLS IS NOT CENTROSOME-ASSOCIATED AND CENTROSOMAL CENTRIN IS CONFINED TO THE DISTAL LUMEN OF CENTRIOLES

Centrin is a member of the calcium-binding EF-hand protein superfamily present in centrosomes of widely divergent species. Investigating the cellular distribution of human centrin by both immunofluorescence and cell fractionation, we report that centrin is biochemically complex i n human cells, displaying as much as ten isoforms in 2-D electrophores is, This suggests that centrin may be subject to multiple regulations, Strikingly, more than 90% of centrin is not associated with the centr osome fraction, The centrosome-associated centrin, however, displays a specific pattern in 2-D electrophoresis and is concentrated within th e distal lumen of the centrioles, where a complex structure has been p reviously described. This precise localization allows the resolution o f centrioles at the optical level throughout the cell cycle and provid es a valuable tool for monitoring centriole duplication. To get insigh ts on centrin function, we performed injection experiments of recombin ant heterologous centrin in two-cell stage frog embryos in an attempt to produce dominant negative effects. We report that green algae and h uman centrin delay cleavage and promote the formation of abnormal blas tomeres in which the distribution of microtubule asters and of nuclei is dramatically impaired, This suggests that centrin could be involved in the centrosome reproduction cycle, in the coordination of cytoplas mic and nuclear division or in cytokinesis.