The MAL protein is crucial for proper membrane condensation at the ciliary base, which is required for primary cilium elongation

Abstract

The base of the primary cilium contains a zone of condensedmembranes whose importance is not known. Here, we have studiedthe involvement of MAL, a tetraspanning protein that exclusivelypartitions into condensed membrane fractions, in the condensation ofmembranes at the ciliary base and investigated the importance ofthese membranes in primary cilium formation. We show that MALaccumulates at the ciliary base of epithelial MDCK cells. Knockdownof MAL expression resulted in a drastic reduction in the condensationof membranes at the ciliary base, the percentage of ciliated cells andthe length of the cilia, but did not affect the docking of the centrosometo the plasma membrane or produce missorting of proteins to thepericentriolar zone or to the membrane of the remaining cilia. Rab8(for which there are two isoforms, Rab8A and Rab8b), IFT88 andIFT20, which are important components of the machinery of ciliarygrowth, were recruited normally to the ciliary base of MAL-knockdowncells but were unable to elongate the primary cilium correctly. MAL,therefore, is crucial for the proper condensation of membranes at theciliary base, which is required for efficient primary cilium extension.