Nucleus-basal body connector in Chlamydomonas: evidence for a role in basal body segregation and against essential roles in mitosis or in determining cell polarity

In the unicellular biflagellate green alga Chlamydomonas reinhardtii each basal body is linked to the nucleus by a fibrous nucleus-basal body connector (NBBC) that contains the calcium-binding protein centrin. (Wright et al.: Journal of Cell Biology 101:1903-1912.; Salisbury et al.: Journal of Cell Biology 107:635-642; Huang et al.: Journal of Cell Biology 107:121-131). In order to explore the cellular function of the NBBC we used antiserum directed against centrin to examine a number of mutants known to be defective for basal body assembly and/or localization. Of three variable flagella-number mutants examined, one, vfl-2, is dramatically defective with respect to the NBBC in that 1) the union between basal bodies and nucleus is very labile, 2) there is no detectible centrin in the NBBC region, and 3) total cellular centrin levels are reduced 75-80% relative to wild type. The existence of these defects in a mutant incapable of maintaining normal flagellar number supports the view that the NBBC plays an important role in determining proper basal body localization and/or segregation. In contrast to vfl-2, the mutants vfl-1, vfl-3, uni-1, and bald-2 contain approximately normal levels of centrin and possess stable NBBCs. The observation of NBBCs in the mutant bald-2, which lacks all but very rudimentary basal bodies, indicates that the assembly of the NBBC does not require fully formed basal bodies and that such assembly may not require basal bodies at all. Finally, the possibility that the NBBC is required for induction of gene expression following deflagellation was tested by examining vfl-2 for such induction. Results indicate that the connector does not play a necessary role in the induction process.