The ODA6 locus of Chlamydomonas reinhardtii encodes a 70 kDa intermediate chain protein of the flagellar outer row dynein ATPase, and mutations at this locus prevent assembly of the entire outer row dynein arm

Dynein ATPases constitute a growing class of microtubuleassociated molecular motors. Although first characterized as force generators in cilia and flagella, dyneins are also cytoplasmic motor proteins with potential roles in axonal transport (Vallee et al., 1988), intracellular organization (Scholey et al., 1984; Lacey and Haimo, 1992) and mitosis (Pfarr et al ., 1990; Steuer et al ., 1990). While a great deal of progress has been made in determining the enzymatic, structural and mechanochemical properties of dyneins, much remains to be learned about the function of individual subunits within these large complexes. This lack of knowledge stems in part from the large size, biochemical complexity and isotypic diversity of these molecules. All dyneins studied to date contain two or three catalytic heavy chains of ~450 kDa, and as many as 12 smaller proteins whose functions are for the most part unknown. While the two heavy chains of some cytoplasmic dyneins may be identical (Vallee et al., 1988), each of the two (Tang et al., 1982) or three (Pfister et al., 1982; Piperno and Luck, 1979) heavy chains of the flagellar dyneins that have been extensively studied are unique proteins, and sequence analysis has shown that an individual dynein heavy chain may have as many as four nucleotide binding site consensus motifs (Gibbons et al., 1991; Ogawa, 1991; Koonce et al., 1992; and D. R. Mitchell, unpublished observations), adding to overall complexity. In Chlamy domonas reinhardtii, outer row dynein arms have three catalytic subunits (alpha, beta and gamma) each containing a unique heavy chain (400-500 kDa) and one or more light chains (14-20 kDa) (Piperno and Luck, 1979; Pfister and Witman, 1984). Each outer row arm also has a non-catalytic subunit consisting of two intermediate chains of ~70 and 78 kDa (IC70 and IC78) and several additional light chains (Pfister and Witman, 1984; Mitchell and Rosenbaum, 1986; see Witman, 1989, 1992, for recent reviews). Results of recent studies suggest that the intermediate chain subunit may play an important role in outer arm structure. The larger intermediate chain, IC78, can be crosslinked directly to tubulin in situ (King et al., 1991), suggesting that it is part of, or in close proximity to, a microtubule attachment site. The smaller intermediate chain, IC70, is required for outer arm assembly (Mitchell and Kang, 1991) and has been localized by immuno-electron microscopy with a monoclonal antibody to the base of 1069 Journal of Cell Science 105,1069-1078 (1993) Printed in Great Britain © The Company of Biologists Limited 1993