Sequence and expression pattern of an evolutionarily conserved transcript identified by gene trapping

Unveiling the intriguing role of FAP20 in tubulin recruitment at the doublet microtubule inner junctions

In this issue of Structure, Bangera et al. investigate the role of the inner junction protein FAP20 in doublet microtubule assembly. Using cryo-EM and microtubule dynamic assays, they demonstrate that FAP20 recruits free tubulins to existing microtubule lattices, shedding light on B-tubule closure during doublet microtubule formation.

FAP20 is an inner junction protein of doublet microtubules essential for both the planar asymmetrical waveform and stability of flagella in Chlamydomonas

The question of what proteins compose the junctions between two tubules in doublet microtubules is long-standing. Here a conserved flagellar protein, FAP20, is shown to be an inner junction component important for stabilizing the axoneme and scaffolding intra–B-tubular structures required for a planar asymmetrical waveform.

The axoneme—the conserved core of eukaryotic cilia and flagella—contains highly specialized doublet microtubules (DMTs). A long-standing question is what protein(s) compose the junctions between two tubules in DMT. Here we identify a highly conserved flagellar-associated protein (FAP), FAP20, as an inner junction (IJ) component. The flagella of Chlamydomonas FAP20 mutants have normal length but beat with an abnormal symmetrical three-dimensional pattern. In addition, the mutant axonemes are liable to disintegrate during beating, implying that interdoublet connections may be weakened. Conventional electron microscopy shows that the mutant axonemes lack the IJ, and cryo–electron tomography combined with a structural labeling method reveals that the labeled FAP20 localizes at the IJ. The mutant axonemes also lack doublet-specific beak structures, which are localized in the proximal portion of the axoneme and may be involved in planar asymmetric flagellar bending. FAP20 itself, however, may not be a beak component, because uniform localization of FAP20 along the entire length of all nine DMTs is inconsistent with the beak's localization. FAP20 is the first confirmed component of the IJ. Our data also suggest that the IJ is important for both stabilizing the axoneme and scaffolding intra–B-tubular substructures required for a planar asymmetrical waveform.

Bug22p, a conserved centrosomal/ciliary protein also present in higher plants, is required for an effective ciliary stroke in Paramecium

Centrioles, cilia, and flagella are ancestral conserved organelles of eukaryotic cells. Among the proteins identified in the proteomics of ciliary proteins in Paramecium, we focus here on a protein, Bug22p, previously detected by cilia and basal-body high-throughput studies but never analyzed per se. Remarkably, this protein is also present in plants, which lack centrioles and cilia. Bug22p sequence alignments revealed consensus positions that distinguish species with centrioles/cilia from plants. In Paramecium, antibody and green fluorescent protein (GFP) fusion labeling localized Bug22p in basal bodies and cilia, and electron microscopy immunolabeling refined the localization to the terminal plate of the basal bodies, the transition zone, and spots along the axoneme, preferentially between the membrane and the microtubules. RNA interference (RNAi) depletion of Bug22p provoked a strong decrease in swimming speed, followed by cell death after a few days. High-speed video microscopy and morphological analysis of Bug22p-depleted cells showed that the protein plays an important role in the efficiency of ciliary movement by participating in the stroke shape and rigidity of cilia. The defects in cell swimming and growth provoked by RNAi can be complemented by expression of human Bug22p. This is the first reported case of complementation by a human gene in a ciliate.

Specific maternal transcripts in bovine oocytes and cleavaged embryos: Identification with novel DDRT-PCR methods

We used annealing control primer (ACP)-based differential display reverse transcription polymerase chain reaction (DDRT-PCR) to isolate differentially expressed amplicons in bovine germinal vesicle (GV) stage oocytes, 8-cell stage embryos produced in vitro, and blastocyst stage embryos produced in vitro. Four expressed sequence tags (ESTs) of genes that were specifically and predominantly expressed in GV oocytes were cloned and sequenced. We have used a fluorescence monitored real-time quantitative PCR (qPCR) to quantify and analyzed the temporal expression of the target differentially expressed transcripts throughout the preimplantation stages from oocytes to blastocysts. The cloned genes or ESTs all exhibited significant sequence similarity with known bovine genes (98%-100%; DNCL1 and ZP2) or ESTs (81%-97%; FANK1 and GTL3) of other species. As revealed by real-time qRT-PCR, DNCL1, FANK1, GTL3, and ZP2 transcripts were observed in the GV stage oocytes and expression gradually decreased up to the 8-cell stage embryo and the transcripts were not detected in later stages. Similarly, upregulation was observed in GV stage mouse oocytes and metaphase II, suggesting that these four differentially expressed orthologous genes play important roles in early preimplantation, as maternally-derived transcripts.

Gene trap: a way to identify novel genes and unravel their biological function

The gene trap methodology is a powerful tool to characterize novel genes and analyze their importance in biological phenomena. It is based on the use of mouse embryonic stem cells and reporter vectors designed to randomly integrate into the genome, tagging an insertion site and generating a mutation. Theoretically, all the 100,000 genes present in the mouse genome could be tagged and functionally inactivated at the same time. Here we describe the basic concepts and perspectives of this methodology and show some results obtained by the gene trap approach used to study molecular cascades in basic cell biology and in developmental processes.