We identified Xenopus pericentriolar material-1 (PCM-1), which had been reported to constitute pericentriolar material, cloned its cDNA, and generated a specific pAb against this molecule. Immunolabeling revealed that PCM-1 was not a pericentriolar material protein, but a specific component of centriolar satellites, morphologically characterized as electron-dense granules, ∼70–100 nm in diameter, scattered around centrosomes. Using a GFP fusion protein with PCM-1, we found that PCM-1–containing centriolar satellites moved along microtubules toward their minus ends, i.e., toward centrosomes, in live cells, as well as in vitro reconstituted asters. These findings defined centriolar satellites at the molecular level, and explained their pericentriolar localization. Next, to understand the relationship between centriolar satellites and centriolar replication, we examined the expression and subcellular localization of PCM-1 in ciliated epithelial cells during ciliogenesis. When ciliogenesis was induced in mouse nasal respiratory epithelial cells, PCM-1 immunofluorescence was markedly elevated at the apical cytoplasm. At the electron microscopic level, anti–PCM-1 pAb exclusively labeled fibrous granules, but not deuterosomes, both of which have been suggested to play central roles in centriolar replication in ciliogenesis. These findings suggested that centriolar satellites and fibrous granules are identical novel nonmembranous organelles containing PCM-1, which may play some important role(s) in centriolar replication.

Adenomatous polyposis coli (APC) tumor suppressor protein has been shown to be localized near the distal ends of microtubules (MTs) at the edges of migrating cells. We expressed green fluorescent protein (GFP)-fusion proteins with full-length and deletion mutants of Xenopus APC in Xenopus epithelial cells, and observed their dynamic behavior in live cells. During cell spreading and wound healing, GFP-tagged full-length APC was concentrated as granules at the tip regions of cellular extensions. At higher magnification, APC appeared to move along MTs and concentrate as granules at the growing plus ends. When MTs began to shorten, the APC granules dropped off from the MT ends. Immunoelectron microscopy revealed that fuzzy structures surrounding MTs were the ultrastructural counterparts for these GFP signals. The COOH-terminal region of APC was targeted to the growing MT ends without forming granular aggregates, and abruptly disappeared when MTs began to shorten. The APC lacking the COOH-terminal region formed granular aggregates that moved along MTs toward their plus ends in an ATP-dependent manner. These findings indicated that APC is a unique MT-associated protein that moves along selected MTs and concentrates at their growing plus ends through their multiple functional domains.

An activity that severs stable microtubules is thought to be involved in microtubule reorganization during the cell cycle. Here, a 48-kilodalton microtubule-severing protein was purified from Xenopus eggs and identified as translational elongation factor 1α (EF-1α). Bacterially expressed human EF-1α also displayed microtubule-severing activity in vitro and, when microinjected into fibroblasts, induced rapid and transient fragmentation of cytoplasmic microtubule arrays. Thus, EF-1α, an essential component of the eukaryotic translational apparatus, appears to have a second role as a regulator of cytoskeletal rearrangements.

Local translation in neuronal dendrites is an important basis for long-term synaptic plasticity, and RNA granules in the dendrites are involved in the local translation. Here, we identify RNG105 (RNA granule protein 105), a novel RNA-binding protein, as a component of the RNA granules in dendrites of hippocampal neurons. The RNG105-localizing RNA granules contain mRNAs, the translational products of which play key roles in synaptic plasticity. RNG105 has an ability to repress translation bothin vitroandin vivo, consistent with the finding that the RNA granule is translationally arrested in the basal conditions. Dissociation of RNG105 from the RNA granules is induced by BDNF, a growth factor responsible for synaptic plasticity. The RNG105 dissociation is coincident with the induction of local translation near the granules. These findings suggest that RNG105 is a translational repressor in the RNA granules and provide insight into the link between RNG105 dynamics and local translational regulation.