The ciliates are model organisms for pioneering studies of ciliary signaling as it controls cell behavior through second messengers such as Ca2+ and cAMP. Signaling is initiated via special receptors uniquely or significantly localized to the ciliary membrane. The receptors initiate a cascade of molecular changes in the ciliary matrix and in certain cases molecules move from the ciliary matrix into the cytoplasm, and sometimes enter the cell nucleus to alter gene expression. Like the cell nucleus, the cilium is specialized compartment of the cytoplasm. The entrance to the ciliary membrane or matrix is defined by the ciliary necklace barriers- ciliary pores-whose composition and function seem related to the nuclear pores. A well-defined signaling cascade can be traced from the cilia-localized receptor tyrosine kinase (TtPTK1) (NEK1) in Tetrahymena. In this cascade, Rad 51 which localizes to both the cilia and the nucleus, presumably may shuttle from the cilium to the nucleus to affect DNA replication and repair. The signaling scaffold protein parafusin (PFUS) may also represent this class of molecules. PFUS is localized to Paramecium dense core secretory vesicles (DSCVs) but also to the cilium and the nucleus. Knockdown of PFUS shuts off overall DSCV production, suggesting an effect on gene expression. Localization of signaling molecules such as Rad 51, Rsk and PFUS to both cilium and nucleus is found in both ciliates and mammalian cells, but the dynamics of movement between compartments is generally unknown and needs further elucidation.
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