Src-family tyrosine kinases (SFKs), which are non-receptor-type tyrosine kinases, consist of proto-oncogene products and structurally related proteins and include at least eight highly homologous proteins: Src, Lyn, Fyn, Yes, Fgr, Hck, Lck and Blk (Brown and Cooper, 1996;Thomas and Brugge, 1997). SFKs are activated by various stimuli, including growth factors and adhesion proteins, and are involved in a wide range of signaling events at the plasma membrane, resulting in cell proliferation, differentiation, migration, and cell-shape changes. Src, Yes, Lyn and Fyn are widely expressed in a variety of cell types, whereas Blk, Fgr, Hck and Lck are found primarily in hematopoietic cells (Bolen and Brugge, 1997;Thomas and Brugge, 1997).SFKs are composed of (1) an N-terminal Src homology (SH) 4 domain that contains lipid modification sites; (2) a poorly conserved 'unique' domain; (3) an SH3 domain that can bind to specific proline-rich sequences; (4) an SH2 domain that can bind to specific sites of tyrosine phosphorylation; (5) an SH1 tyrosine kinase catalytic domain; and (6) a negative regulatory tail for autoinhibition of kinase activity (Brown and Cooper, 1996;Thomas and Brugge, 1997). All members of the Src family are cotranslationally myristoylated at Gly2 and, with the exception of Src and Blk, are also post-translationally palmitoylated at Cys3, Cys5 or Cys6 (Paige et al., 1993;Alland et al., 1994;Koegl et al., 1994;Resh, 1994; Shenoy-Scaria et al., 1994; Kasahara et al., 2007a). Fatty acylation of SFKs has been shown to influence their interactions with cell membranes (McCabe and Berthiaume, 1999;Resh, 1999) and as a consequence their intracellular distribution.It is generally thought that SFKs are predominantly located at the cytoplasmic face of the plasma membrane through posttranslational myristoylation, usually with subsequent palmitoylation, but in fact, appreciable fractions are found at in variety of intracellular locations, such as endosomes, secretory granules or phagosomes and the Golgi complex (Kaplan et al., 1992;Mohn et al., 1995;Brown and Cooper, 1996;Thomas and Brugge, 1997;Kasahara et al., 2004). Although distinctive localizations of SFK members have been implicated in their specific functions, the mechanism that underlies the targeting of SFKs to their specific locations remains to be elucidated.We recently showed that Lyn, a palmitoylated SFK, is exocytosed to the plasma membrane via the Golgi region along the secretory pathway (Kasahara et al., 2004). More recently, we demonstrated that Src, a non-palmitoylated SFK, rapidly moves between the plasma membrane and late endosomes or lysosomes, and that mutation of Cys3 in Lyn allows Lyn to traffic in a similar manner to Src (Kasahara et al., 2007a), indicating the importance of palmitoylation for distinct trafficking between Lyn and Src.In this study, we investigate the localization and trafficking of other ubiquitously expressed SFKs, such as Yes and Fyn. We demonstrate that Lyn and Yes, which are monopalmitoylated SFKs, Src-family tyrosine kin...

Primary cilia, microtubule-based sensory structures, orchestrate various critical signals during development and tissue homeostasis. In view of the rising interest into the reciprocal link between ciliogenesis and cell cycle, we discuss here several recent advances to understand the molecular link between the individual step of ciliogenesis and cell cycle control. At the onset of ciliogenesis (the transition from centrosome to basal body), distal appendage proteins have been established as components indispensable for the docking of vesicles at the mother centriole. In the initial step of axonemal extension, CP110, Ofd1, and trichoplein, key negative regulators of ciliogenesis, are found to be removed by a kinase-dependent mechanism, autophagy, and ubiquitin–proteasome system, respectively. Of note, their disposal functions as a restriction point to decide that the axonemal nucleation and extension begin. In the elongation step, Nde1, a negative regulator of ciliary length, is revealed to be ubiquitylated and degraded by CDK5-SCFFbw7 in a cell cycle-dependent manner. With regard to ciliary length control, it has been uncovered in flagellar shortening of Chlamydomonas that cilia itself transmit a ciliary length signal to cytoplasm. At the ciliary resorption step upon cell cycle re-entry, cilia are found to be disassembled not only by Aurora A-HDAC6 pathway but also by Nek2-Kif24 and Plk1-Kif2A pathways through their microtubule-depolymerizing activity. On the other hand, it is becoming evident that the presence of primary cilia itself functions as a structural checkpoint for cell cycle re-entry. These data suggest that ciliogenesis and cell cycle intimately link each other, and further elucidation of these mechanisms will contribute to understanding the pathology of cilia-related disease including cancer and discovering targets of therapeutic interventions.

Primary cilia are microtubule-based sensory organelles that organize numerous key signals during developments and tissue homeostasis. Ciliary microtubule doublet, named axoneme, is grown directly from the distal end of mother centrioles through a multistep process upon cell cycle exit; however, the instructive signals that initiate these events are poorly understood. Here we show that ubiquitin-proteasome machinery removes trichoplein, a negative regulator of ciliogenesis, from mother centrioles and thereby causes Aurora-A inactivation, leading to ciliogenesis. Ciliogenesis is blocked if centriolar trichoplein is stabilized by treatment with proteasome inhibitors or by expression of non-ubiquitylatable trichoplein mutant (K50/57R). Started from two-stepped global E3 screening, we have identified KCTD17 as a substrate-adaptor for Cul3-RING E3 ligases (CRL3s) that polyubiquitylates trichoplein. Depletion of KCTD17 specifically arrests ciliogenesis at the initial step of axoneme extension through aberrant trichoplein-Aurora-A activity. Thus, CRL3-KCTD17 targets trichoplein to proteolysis to initiate the axoneme extension during ciliogenesis.