Dartmouth researchers Roger Sloboda, Megan Ulland, and Mark Schneider identified a cobalamin (vitamin B12) independent form of the enzyme methionine synthase (MetE) in Chlamydomonas flagella as well as four methylated proteins in resorbing flagella. These discoveries show that a protein methylation pathway is active during flagellar resorption and, because flagella are resorbed prior to cell division, suggest a link between flagellar protein methylation and progression through the cell cycle. The finding was reported last month in Molecular Biology of the Cell.
Flagella and cilia, which have identical internal structure in eukaryotes, are important for cell movement and sensory signaling. The study of flagella has been gaining momentum in recent years, with the discovery of several new functions and cellular processes in which they are now known to play key roles. An increasing number of genetic diseases can now be linked to mutations in flagellar genes; among these are polypeptides required for intraflagellar transport (IFT), a relatively recent discovery in itself.
Intraflagellar transport involves the bidirectional movement of proteins along the length of the flagellum or cilium. This process is crucial because it is responsible for the generation and maintenance of flagella and cilia and is involved in signal transduction and transcriptional control. A key area of interest in IFT research is the flagellar tip complex (FTC), the region where anterograde IFT (transport away from the cell body) stops and retrograde IFT (transport back to the cell body) begins.
Initially, Sloboda, Ulland, and Schneider focused on identifying proteins localized to the FTC. In the process, they discovered that a cobalamin-independent form of methionine synthase (MetE) had a higher concentration in regenerating flagella compared to full-length flagella. However, through further analysis, they found MetE was distributed along the length of the flagellum and was therefore not an FTC protein.
Surprisingly, MetE was found to be associated with some, but not all, IFT particles. MetE catalyzes the conversion of homocysteine to methionine, which is then converted to S-adenosyl methionine (SAM). SAM acts as a methyl donor and is therefore necessary for all protein methylations, suggesting a protein methylation pathway required for certain phases of flagellar growth or maintenance. Four proteins were then identified that were specifically methylated in the process of flagella being resorbed, showing that the protein methylation path was active specifically during flagellar resorption.
Methylation is important because it regulates cell activity through posttranslational modification. However, the specific importance of protein methylation in flagellar resorption is not yet known. Protein methylation may either alter key protein interactions or create protein interactions required for flagellar disassembly. Sloboda’s lab is currently working to identify the genes that produced the four methylated proteins in the cell and eventually determine their role in flagellar resorption.
Since resorption of flagella precedes entry into the cell cycle, the research indicates that flagellar protein methylation may send a signal that regulates cellular division. However, further studies are required to establish this link.