A NOVEL BASAL APPARATUS PROTEIN OF 90 KD (BAP90) FROM THE FLAGELLATE GREEN-ALGA SPERMATOZOPSIS-SIMILIS IS A COMPONENT OF THE PROXIMAL PLATES AND IDENTIFIES THE D-(DEXTER)SURFACE OF THE BASAL BODY

The flagellar basal apparatus consists of the basal bodies and associa ted fibrous structures, and represents the organizing center for the m icrotubular cytoskeleton in many flagellate protists. To identify nove l proteins associated with the basal bodies, basal apparatuses from th e flagellate green alga Spermatozopsis similis (Chlorophyceae) were is olated and purified, A polyclonal antibody raised against a 95 kD prot ein band enriched in purified basal apparatuses was used to screen a c DNA library of S. similis which resulted in the isolation of a full le ngth clone coding for a novel protein of 812 amino acids (85.3 kD), Se quence analysis of this clone identified extended alpha-helical region s and predicted several coiled-coil forming domains interrupted by spa cer segments of variable lengths. A polyclonal antibody (anti-BAp90) r aised against the bacterially expressed protein recognized a 90 kD ban d (BAp90) in SDS-PAGE of isolated basal apparatuses of S. similis. Imm unogold labeling using anti-BAp90 decorated the proximal plates (two s triated, triangular fibers which serve as spacers between the basal bo dies in their proximal region) and parts of the d-fibers (df) which in terconnect the basal bodies with the microtubular d-roots and the stri ated microtubule-associated fibers (SMAFs), Thus, the 90 kD basal appa ratus protein of S. similis represents a biochemical landmark for the lateral asymmetry of the basal body identifying its d-(dexter)surface. Cytoskeletal elements containing BAp90 or structurally related protei ns may be involved in the organization and/or maintenance of the posit ional relationship between basal bodies and the cellular cytoskeleton, and hence cell polarity.