Non-membranous granular organelle consisting of PCM-1: subcellular distribution and cell-cycle-dependent assembly/disassembly

Centriolar satellites were initially identified as electrondense spherical granules, approximately 70-100 nm in diameter, localized around the centrosomes. We have previously identified pericentriolar material 1 (PCM-1), with a molecular mass of approximately 230 kDa, as a component of centriolar satellites. We now show by immunofluorescence microscopy that these granules are not only concentrated around centrioles but also scattered throughout the cytoplasm in various types of mouse cells, leading us tentatively to call them 'PCM-1 granules'. We then found that, when overexpressed, PCM-1 molecules lacking their C-terminal region bound directly with each other through two distinct regions to form large aggregates, which then recruited endogenous PCM-1. These large aggregates as well as endogenous PCM-1 granules appear to be disassembled during mitosis, and reassembled when the cells entered interphase. These findings suggest that PCM-1 granules are formed by self-aggregation of PCM-1 and that this self-aggregation is regulated in a cell-cycle-dependent manner. Furthermore, we found that PCM-1 granules are distinct from pericentrin-containing granules, and that these two distinct types of granular structures are frequently associated with each other within the cytoplasm. These findings are discussed with special reference to the possible physiological functions of PCM-1 granules.