HIGHLY DIVERGENT ACTIN EXPRESSED IN A CHLAMYDOMONAS MUTANT LACKING THE CONVENTIONAL ACTIN GENE

The Chlamydomonas mutant ida5 is deficient in the conventional actin g ene and its axoneme lacks a subset of inner dynein arms that contain a ctin as a subunit. However, this mutant retains some other inner dynei n arms because a novel protein (NAP) is expressed as a substitute for actin. In this study, we show by sequence analysis that NAP is identic al to a putative actin-related protein, the cDNA sequence of which has recently been reported and shown to have 64% amino acid identity with conventional actin. A polyclonal antibody raised against a synthetic polypeptide corresponding to the NH2-terminal sequence of this protein specifically reacted with the spot corresponding to NAP in two-dimens ional electrophoresis patterns. NAP apparently can substitute for conv entional actin in some, but not all, cellular functions, and therefore can be regarded as a highly divergent actin. This unconventional acti n appears to be expressed only when conventional actin is absent. (C) 1998 Academic Press.