Reduction of meckelin leads to general loss of cilia, ciliary microtubule misalignment and distorted cell surface...

Meckelin (MKS3), a conserved protein linked to Meckel Syndrome, assists in the migration of centrioles to the cell surface for ciliogenesis. We explored for additional functions of MKS3p using RNA interference (RNAi) and expression of FLAG epitope tagged protein in the ciliated protozoan Paramecium tetraurelia. This cell has a highly organized cell surface with thousands of cilia and basal bodies that are grouped into one or two basal body units delineated by ridges. The highly systematized nature of the P. tetraurelia cell surface provides a research model of MKS and other ciliopathies where changes in ciliary structure, subcellular organization and overall arrangement of the cell surface can be easily observed. We used cells reduced in IFT88 for comparison, as the involvement of this gene's product with cilia maintenance and growth is well understood.

FLAG-MKS3p was found above the plane of the distal basal body in the transition zone. Approximately 95% of those basal bodies observed had staining for FLAG-MKS3. The RNAi phenotype for MKS3 depleted cells included global shortening and loss of cilia. Basal body structure appeared unaffected. On the dorsal surface, the basal bodies and their associated rootlets appeared rotated out of alignment from the normal anterior-posterior rows. Likewise, cortical units were abnormal in shape and out of alignment from normal rows. A GST pull down using the MKS3 coiled-coil domain suggests previously unidentified interacting partners.

Reduction of MKS3p shows that this protein affects development and maintenance of cilia over the entire cell surface. Reduction of MKS3p is most visible on the dorsal surface. The anterior basal body is attached to and moves along the striated rootlet of the posterior basal body in preparation for duplication. We propose that with reduced MKS3p, this attachment and guidance of the basal body is lost. The basal body veers off course, causing basal body rows to be misaligned and units to be misshapen. Rootlets form normally on these misaligned basal bodies but are rotated out of their correct orientation. Our hypothesis is further supported by the identification of novel interacting partners of MKS3p including a kinetodesmal fiber protein, KdB2.