ABSTRACT
Striated fiber assemblin (SF-assemblin or SFA) is the major component of the striated microtubule-associated fibers (SMAFs) in the flagellar basal apparatus of green flagellates. We generated nuclear transformants of Chlamydomonas expressing green fluorescent protein (GFP) fused to the C-terminus of SFA. SFA-GFP assembled into striated fibers that exceeded those of wild-type cells in size by several fold. At elevated temperatures (>or=32 degrees C) SFA-GFP was mostly soluble and heat shock depolymerized the SMAFs. C-terminal deletions of 18 or only six residues disturbed the ability of SFA-GFP to polymerize, indicating an important role of the C-terminal domain for fiber formation. The exchange of the penultimate Ser275 with alanine made SFA-GFP highly insoluble, causing aberrant fiber formation and conferring heat stability to the fibers. By contrast, a replacement with glutamic acid increased the solubilty of the molecule, indicating that phosphorylation on Ser275 might control solubility of SFA. In vivo observation of GFP fluorescence showed that SFA-GFP fibers were disassembled during mitosis. In cells overexpressing full-length or truncated SFA-GFP, the amount of wild-type protein was reduced. Elevated temperatures dissolved SFA-GFP fibers and induced the synthesis of SFA, suggesting that cells control both the amount of soluble and polymeric SFA. By expressing constructs consisting of cDNA and genomic DNA for parts of SFA in antiparallel configuration, the amount of SFA was severely reduced. In these strains we observed defects in flagellar assembly, indicating an important role for noncontractile striated roots in the flagella apparatus.
