THE SUP-PF-2 MUTATIONS OF CHLAMYDOMONAS ALTER THE ACTIVITY OF THE OUTER DYNEIN ARMS BY MODIFICATION OF THE GAMMA-DYNEIN HEAVY-CHAIN

The sup-pf-2 mutation is a member of a group of dynein regulatory muta tions that are capable of restoring motility to paralyzed central pair or radial spoke defective strains. Previous work has shown that the f lagellar beat frequency is reduced in sup-pf-2, but little else was kn own about the sup-pf-a phenotype (Huang, B., Z. Ramanis, and D.J.L. Lu ck, 1982. Cell. 28:115-125; Brokaw, C.J., and D.J.L, Luck. 1985, Cell Motil. 5:195-208), We have reexamined sup-pf-2 using improved biochemi cal and structural techniques and by the analysis of additional sup-pf -2 alleles. We have found that the sup-pf-2 mutations are associated w ith defects in the outer dynein arms. Biochemical analysis of sup-pf-2 -1 axonemes indicates that both axonemal ATPase activity and outer arm polypeptides are reduced by 40-50% when compared with wild type. By t hin-section EM, these defects correlate with an not similar to 45% los s of outer dynein arm structures. Interestingly, this loss is biased t oward a subset of outer doublets, resulting in a radial asymmetry that may reflect some aspect of outer arm assembly. The defects in outer a rm assembly do not appear to result from defects in either the outer d oublet microtubules or the outer arm docking structures, but rather ap pear to result from defects in outer dynein arm components, Analysis o f new sup-pf-2 mutations indicates that the severity of the outer arm assembly defects varies with different alleles. Complementation tests and linkage analysis reveal that the sup-pf-2 mutations are alleles of the PF28/ODA2 locus, which is thought to encode the gamma-dynein heav y chain subunit of the outer arm. The sup-pf-2 mutations therefore app ear to alter the activity of the outer dynein arms by modification of the gamma-dynein heavy chain.