BRAIN CYTOPLASMIC AND FLAGELLAR OUTER ARM DYNEINS SHARE A HIGHLY CONSERVED M(R) 8,000 LIGHT-CHAIN

Sequence comparisons with the M(r) 8,000 light chain from Chlamydomona s outer arm dynein revealed the presence of highly conserved homologue s (up to 90% identity) in the expressed sequence tag data base (King, S. M. & Patel-King, R. S. (1995a) J. Biol. Chem. 270, 11445-11452). Se veral of these homologous sequences were derived from organisms and/or tissues that lack motile cilia/flagella, suggesting that these protei ns may function in the cytoplasm. In Drosophila, lack of the homologou s protein results in embryonic lethality (Dick, T., Ray, K., Salz, H. K. & Chia, W. (1996) Mol. Cell. Biol., 16, 1966-1977). Fractionation o f mammalian brain homogenates reveals three distinct cytosolic pools o f the homologous protein, one of which specifically copurifies with cy toplasmic dynein following both ATP-sensitive microtubule affinity/suc rose density gradient centrifugation and immunoprecipitation with a mo noclonal antibody specific for the 74-kDa intermediate chain (IC74). Q uantitative densitometry indicates that there is one copy of the M(r) 8,000 polypeptide per IC74. Dual channel confocal immunofluorescent mi croscopy revealed that the M(r) 8,000 protein is significantly colocal ized with cytoplasmic dynein but not with kinesin in punctate structur es (many of which are associated with microtubules) within mammalian o ligodendrocytes. Thus, it appears that flagellar outer arm and brain c ytoplasmic dyneins share a highly conserved light chain polypeptide th at, at least in Drosophila, is essential for viability.