Regulation of flagellar assembly and length in Chlamydomonas by LF4, a MAPK-related kinase.

Members of the MAPK superfamily are known as key regulators of ciliogenesis. Long flagellar (LF) 4, a MAPK-related kinase in Chlamydomonas, is the first kinase that was implicated in ciliary assembly and length. However, little is known about its cellular properties, regulation, and molecular functions. LF4 is localized both in the flagella and cell body with enrichment at the 2 basal bodies, shown by super-resolution microscopy. LF4 is constitutively phosphorylated at T159 at the kinase activation loop and remains at the basal bodies during flagellar assembly. Gene mutations that affect the kinase activity or T159 phosphorylation alter the localization of LF4 at the basal bodies, and the mutants fail to rescue lf4-3, a null mutant. LF4 does not affect the velocities of intraflagellar transport (IFT). However, LF4 null mutation induces accumulation of IFT proteins in the flagellum and reduces the phosphorylation of the kinesin-II subunit FLA8/KIF3B, indicating that LF4 negatively regulates IFT entry. Furthermore, LF2, a cell cycle-related kinase, and LF3, a novel protein, are required for LF4 phosphorylation. Our study demonstrates that LF4 is likely a constitutively active kinase that is regulated by LF2 and regulates IFT entry at the basal bodies to control flagellar assembly and length.-Wang, Y., Ren, Y., Pan, J. Regulation of flagellar assembly and length in Chlamydomonas by LF4, a MAPK-related kinase.