Members of the NIMA-related Kinase Family Promote Disassembly of Cilia by Multiple Mechanisms

Abstract

The genome of Tetrahymena thermophila contains 39 loci encoding NIMA-related kinases (NRKs), an extraordinarily large number for a unicellular organism. Evolutionary analyses grouped these sequences into several subfamilies, some of which have orthologues in animals, whereas others are protist specific. When overproduced, NRKs of three subfamilies caused rapid shortening of cilia. Ultrastructural studies revealed that each NRK triggered ciliary resorption by a distinct mechanism that involved preferential depolymerization of a subset of axonemal microtubules, at either the distal or proximal end. Overexpression of a kinase-inactive variant caused lengthening of cilia, indicating that constitutive NRK-mediated resorption regulates the length of cilia. Each NRK preferentially resorbed a distinct subset of cilia, depending on the location along the anteroposterior axis. We also show that normal Tetrahymena cells maintain unequal length cilia. We propose that ciliates used a large number of NRK paralogues to differentially regulate the length of specific subsets of cilia in the same cell. INTRODUCTIONHow the size of organelles is determined is an important and largely unanswered question. Cilia have emerged as a model organelle to study the mechanism of size regulation (Marshall, 2002). The size of an organelle is determined by the balance between assembly pathways that deliver structural components and disassembly pathways that remove components. In cilia, the intraflagellar transport (IFT), a bidirectional motility system that operates inside cilia (Rosenbaum and Witman, 2002;Scholey, 2003), plays a key role in length regulation. The anterograde IFT supplies structural subunits during assembly (Piperno et al., 1996;Qin et al., 2005) and maintains the axoneme after assembly (Brown et al., 1999). The retrograde IFT recycles the IFT machinery (Signor et al., 1999) and removes ciliary structural components that turn over (Qin et al., 2004). This turnover is the basis of the disassembly pathway (Stephens, 1997;Marshall and Rosenbaum, 2001;Song and Dentler, 2001;Thazhath et al., 2004). Experimental data and mathematical modeling in Chlamydomonas reinhardtii indicate that the length of flagella is dependent on the rates of elongation (equivalent to the efficiency of anterograde IFT) and disassembly. As the flagellum grows, the rate of elongation decreases due to the limited supply of IFT components that need to travel longer distances to the tip (Marshall and Rosenbaum, 2001). According to the "balance point model," the steady-state length is achieved when the rate of elongation equals the rate of disassembly (Marshall and Rosenbaum, 2001;Marshall et al., 2005).The length of already assembled cilia can be reduced drastically using two mechanisms: autotomy or resorption (reviewed by Quarmby, 2004). During autotomy, the axoneme breaks off from the basal body, the cilium falls off, and the plasma membrane reseals over the basal body. Protists undergo autotomy in response to low pH (Lewin et al., 1982). It is unclear whether...

Cell cycle regulation by the NEK family of protein kinases

“…NEK2 is a core component of the human centrosome (Andersen et al, 2003;Fry et al, 1998b), where it regulates a key step in the centrosome cycle, namely centrosome disjunction (O'Regan et al, 2007). Importantly, many NEKs, from fungi to humans, are found at the respective microtubule-organising centres (MTOCs) (for examples, see De Souza et al, 2000;Krien et al, 2002;Mahjoub et al, 2002;Prigent et al, 2005;Wloga et al, 2006). This crucial piece of evidence has led to the hypothesis that NEKs exert their functions through regulation of centrosomes and the microtubule structures that they organise, and that this might not be restricted to dividing cells (Box 1).…”

“…Roles for NEKs in cilia were first revealed through studies in ciliated unicellular eukaryotes, such as Chlamydomonas and Tetrahymena (Bradley and Quarmby, 2005;Mahjoub et al, 2002;Wloga et al, 2006). Compared with non-ciliated fungi, the complement of genes encoding NEKs in these organisms has expanded considerably, and functional studies have suggested that these NEKs might be crucial in regulating ciliary length (Parker et al, 2007).…”

The Toxoplasma gondii centrosome is the platform for internal daughter budding as revealed by a Nek1 kinase mutant

“…For instance, in the ciliate Tetrahymena thermophila an extended family of 39 Neks controls cilium length and stability (Wloga et al, 2006). Three NIMA kinases named NRK were identified in Trypanosoma brucei, the parasite responsible for sleeping sickness in human and animals.…”

Nek8 Regulates the Expression and Localization of Polycystin-1 and Polycystin-2

“…1,2 NIMA-related kinase (Nek or Nrk) of Tetrahymena thermophila, Nrk-1 and Nrk-2, localize to the basal bodies and cilia and regulate ciliary length. 3 Fa2p is a NIMA homolog from Chlamydomonas reinhardtii and has been shown to have roles in cell cycle progression and microtubule severing during deflagellation. 4 Recently, Fa2p was found localized to the proximal end of cilia, in both Chlamydomonas and cultured kidney epithelial cells, and its kinase activity is required for deflagellation.…”