With the recent release of version 24 of the Human Protein Atlas we provide a detailed map of 654 proteins that localize to a small, but important organelle, called the primary cilium. This tiny protrusion is present on most human cell types and serves as a cellular antenna, providing a compartment that is optimal for carefully sensing, transducing and relaying information from the extracellular environment.

Our detailed mapping effort clearly demonstrates that the proteome of primary cilia is highly dynamic and versatile. Not only is the set of proteins present in primary cilia cell-type specific, but there is a fascinating degree of cell-to-cell variability also within populations of the same cell type. This indicates that the signaling pathways of primary cilia can be rapidly and carefully adopted to fit the current need of the cell. However, the high degree of heterogeneity, combined with the fact that many ciliary proteins also localize to other subcellular compartments, makes it difficult to find standardized marker proteins for visualising primary cilia in imaging assays. This is in turn important for further exploring the ciliary proteome of individual cells in different cell types, tissues and contexts.

Among the 654 ciliary proteins in the subcellular resource the only protein that stably and exclusively localize to primary cilia across all three cell lines tested is CFAP300. The function of this protein is not known, but mutations in CFAP300 have been recently reported in patients with motile ciliopathy phenotypes and abnormal motile cilia structure. Indeed, from the immunohistochemial (IHC) stainings in the tissue resource, we can see that CFAP300 localizes to primary cilia of several tissues, as well as to motile cilia of the fallopian tube. Thus, CFAP300 emerges as a good candidate for serving as a standard marker for cilia, which we hope may aid the research community in the quest to further explore the structure and function of this intriguing organelle.

Read more in the preprint by Hansen et al.

Ulrika Axelsson