THE BOP2-1 MUTATION REVEALS RADIAL ASYMMETRY IN THE INNER DYNEIN ARM REGION OF CHLAMYDOMONAS-REINHARDTII

Strains of Chlamydomonas reinhardtii with a mutant allele at the BOP2 locus swim slowly and have an abnormal flagellar waveform similar to p reviously identified strains with defects in the inner arm region. Dou ble mutant strains with the bop2-1 allele and any of 17 different muta tions that affect the dynein arm region swim more slowly than either p arent, which suggests that the bop2-1 mutation does not affect solely the outer dynein arms, the Il or ida4 inner dynein arms, or the dynein regulatory complex. Flagellar axonemes isolated from bop2-1 cells are missing a phosphorylated polypeptide of 152 kD. Electron microscopic analysis shows that bop2-1 axonemes are missing density in the inner d ynein arm region. Surprisingly, two populations of images were observe d in longitudinal sections of axonemes from the bop2-1 strain. In the 10 longitudinal axonemes examined, a portion of the dynein regulatory complex and a newly identified structure, the projection, are affected . In five of these 10 longitudinal axonemes examined, two lobes of the ida4 inner arm are also missing. By examining the cross-sectional ima ges of wild-type and bop2-1 axonemes at each outer doublet position ar ound the axoneme, we have determined that the bop2-1 mutation affects the assembly of inner arm region components in a doublet specific mann er. Doublets 5, 6, and 8 have the most severe deficiency, doublet 9 ha s an intermediate phenotype, and doublets 2, 3, 4, and 7 have the leas t severe phenotype. The bop2-1 mutation provides the first evidence of radial asymmetry in the inner dynein arm region.