TetrahymenaPoc5 is a transient basal body component that is important for basal body maturation

Basal bodies (BBs) are microtubule-based organelles that template and stabilize cilia at the cell surface. Centrins ubiquitously associate with BBs and function in BB assembly, maturation, and stability. Human POC5 (hPOC5) is a highly conserved centrin-binding protein that binds centrins through Sfi1p-like repeats and is required for building full-length, mature centrioles. Here, we use the BB-rich cytoskeleton ofTetrahymena thermophilato characterize Poc5 BB functions.TetrahymenaPoc5 (TtPoc5) uniquely incorporates into assembling BBs and is then removed from mature BBs prior to ciliogenesis. Complete genomic knockout ofTtPOC5leads to a significantly increased production of BBs yet a markedly reduced ciliary density, both of which are rescued by reintroduction of TtPoc5. A secondTetrahymena POC5-like gene,SFR1, is similarly implicated in modulating BB production. WhenTtPOC5andSFR1are co-deleted, cell viability is compromised, and levels of BB overproduction are exacerbated. Overproduced BBs display defective transition zone formation and a diminished capacity for ciliogenesis. This study uncovers a requirement for Poc5 in building mature BBs, providing a possible functional link betweenhPOC5mutations and impaired cilia.SUMMARY STATEMENTLoss ofTetrahymena thermophilaPoc5 reveals an important role for this centrin-binding protein in basal body maturation, which also impacts basal body production and ciliogenesis.