The centrosome is the main microtubule organizing center (MTOC) in human cells, and has been widely studied ever since Theodor Boveri first named and described it in 1888. Although the centrosome is a small organelle, it is of great importance for fundamental cellular functions. Located adjacent to the nucleus, the major role of the centrosome is to regulate the intracellular organization of microtubules. During cell division, the centrosome is the key responsible organelle for the correct formation and orientation of the mitotic spindle, ensuring proper segregation of sister chromatids to each of the daughter cells (Nigg EA et al. (2011)).
In the Cell Atlas, 548 genes (3% of all protein-coding human genes) have been shown to encode proteins that localize to the centrosome or the centriolar satellites. In images where it has been possible to distinguish the centrioles, proteins have been annotated with the location "centrosome". In images where the centrioles have not been detected, but the protein localizes to the center of the microtubules, proteins have been annotated with the location "centriolar satellite" (Figure 1-2). Functional enrichment analysis of the centrosome proteome shows enrichment of terms for biological processes related to intracellular organization and transport, organization of microtubules, cell cycle progression and cell division.
Figure 1. Examples for proteins localized to the centrosome and centriolar satellites. RAB11FIP5 is involved in intracellular transport and has previously not been shown to localize to centrosomes. By using independent antibodies, RAB11FIP5 is localized to centriolar satellites (detected in A-431 cells). PCNT is a well-characterized protein component of the filamentous matrix of the centrosome with important roles in both mitosis and meiosis (detected in U-251 cells). MKKS is a centrosome-shuttling protein that localizes to a tube-like structure around the centrioles in the pericentriolar material (PCM) and is important for cell division (detected in U-2 OS cells). Normally, MKKS shuttles between the centrosome and the cytosol throughout the cell cycle but when mutated, it fails to localize to the centrosome, leading to the McKusick Kaufman syndrome, a disease that manifests with impaired development, especially of hands and feet, as well as heart and genital defects.
Figure 2. 3% of all human protein-coding genes encode proteins localized to the centrosome or the centriolar satellites. Each bar is clickable and gives a search result of proteins that belong to the selected category.
The structure of the centrosome
The centrosome is a small non-membrane bound organelle occupying about 1-2 μm3 of the cytoplasmic volume (Doxsey S. (2001)). It is composed of two barrel shaped centrioles, each composed of nine microtubule triplets, held together at a perpendicular angle by interconnecting fibers. The centrioles are surrounded by an amorphous matrix of proteins, commonly referred to as the pericentriolar material (PCM), which contains proteins involved in nucleation and anchoring of microtubules, as well as important cell cycle regulators and other signaling molecules. Pericentrin (Figure 1), γ-tubulin, ninein, centriolin and aurora kinases are some examples (Doxsey S. (2001)).The γ-tubulin protein complex is a highly conserved complex that forms an open ring structure, around 25 nm in diameter, and plays a key role in nucleation of microtubules. As a key regulator of mitosis, the structure and composition of the centrosome is highly dynamic and undergoes dramatic organizational changes throughout the cell cycle (Bornens M. (2002); Conduit PT et al. (2015)).
The centrosomes, as well as the basal body of cilia, is closely surrounded by cytoplasmic granules, known as centriolar satellites (Tollenaere MA et al. (2015); Prosser SL et al. (2020)). Centriolar satellites travel along microtubules by association with motor proteins and are known to contain a number of proteins that are also found in centrosomes and cilia. Centriolar satellites can be observed in most cell types, but their composition, size, number and location varies. Centriolar satellites disassemble upon entry into mitosis, bur reappear upon completion of cytokinesis.
A selection of proteins suitable as markers for the centrosome and the centriolar satellites can be found in Table 1. A list of highly expressed proteins that localize to centrosomes and centriolar satellites are summarized in Table 2.
Table 1. Selection of proteins suitable as markers for the centrosome and centriolar satellites.
Table 2. Highly expressed centrosome and centriolar satellite marker proteins, in different cell lines.
See the morphology of centrosomes in human induced stem cells in the Allen Cell Explorer.
The function of the centrosome
The major function of the centrosome is organization of microtubules in the cell, thereby controlling cellular shape, polarity, proliferation, mobility and cell division. During S-phase, the centrosome is replicated in a semi-conservative manner, resulting in formation of one daughter centriole next to each of the parental centrioles. As the cell approaches mitosis, the two centrosomes, each containing a parental centriole and a maturing procentriole, move to opposite ends of the cell. At the same time, the amount of surrounding PCM proteins increase, enabling nucleation of more microtubules. When the nuclear membrane breaks down, microtubules originating from each of the centrosomes can interact with kinetochores on the replicated sister chromatids, forming the characteristic mitotic spindle. The intricate spindle apparatus mediates separation of sister chromatids to opposite ends of the cell, and upon cytokinesis each of the daughter cells is provided with one set of chromosomes and one centrosome. The parental centriole, i.e. the older of the two in the centriole pair, also has a central role in formation of cilia and flagella. Moreover, increasing evidence suggest a more versatile function of the centrosome, especially pointing to its ability to coordinate a myriad of cellular functions by serving as a compact hub where cytoplasmic proteins can interact at high concentrations (Doxsey S. (2001); Rieder CL et al. (2001)).
Centriolar satellites have long been considered as vehicles for protein trafficking to and from the centrosome and cilia, thus playing a role in dynamic regulation of protein composition in these organelles (Tollenaere MA et al. (2015); Prosser SL et al. (2020)). Indeed, several proteins that localize to centriolar satellites have been implicated in centrosome replication, maturation and separation. However, in recent studies, centriolar satellites have also emerged as regulators of multiple other cellular processes, such as protein degradation and autophagy, some of which are independent of centrosomes and cilia. Similarly, centrosomes and cilia are not fully dependent on centriolar satellites.
As key regulators of chromosome segregation and cell cycle progression, abnormalities in number, size and morphology of the centrosome, and mutations in genes encoding protein that localize to centrosomes, is commonly observed in cells undergoing tumorigenesis, but also in some other diseases (Badano JL et al. (2005)).
Gene Ontology (GO)-based analysis of genes encoding proteins that localize to centrosomes or centriolar satellites shows enrichment of terms describing functions that are well in-line with existing literature. The most highly enriched terms for the GO domain Biological Process are related to mitosis and cytokinesis, cell cycle progression, endocytosis, organization of the microtubule cytoskeleton, and organization of organelles (Figure 3a). Enrichment analysis of the GO domain Molecular Function reveal enrichment of terms describing binding to microtubules and motor proteins, as well as motor activity (Figure 3b).
Figure 3a. Gene Ontology-based enrichment analysis for the centrosome proteome showing the significantly enriched terms for the GO domain Biological Process. Each bar is clickable and gives a search result of proteins that belong to the selected category.
Figure 3b. Gene Ontology-based enrichment analysis for the centrosome proteome showing the significantly enriched terms for the GO domain Molecular Function. Each bar is clickable and gives a search result of proteins that belong to the selected category.
Centrosome proteins with multiple locations
Approximately 78% (n=428) of the centrosome and centriolar satellite proteins detected in the Cell Atlas also localize to other cellular compartments (Figure 4). The network plot shows that the most common locations shared with the centrosome and centriolar satellites are the cytoplasm, nucleus and vesicles. Dual localizations with nucleoplasm and cytosol are overrepresented, while dual localizations with the Golgi apparatus and nucleoli are underrepresented.
Figure 4. Interactive network plot of microtubule proteins with multiple localizations. The numbers in the connecting nodes show the proteins that are localized to the centrosome and to one or more additional locations. Only connecting nodes containing more than one protein and at least 0.5% of proteins in the centrosome proteome are shown. The circle sizes are related to the number of proteins. The cyan colored nodes show combinations that are significantly overrepresented, while magenta colored nodes show combinations that are significantly underrepresented as compared to the probability of observing that combination based on the frequency of each annotation and a hypergeometric test (p≤0.05). Note that this calculation is only done for proteins with dual localizations. Each node is clickable and results in a list of all proteins that are found in the connected organelles.
Expression levels of centrosome proteins in tissue
Transcriptome analysis and classification of genes into tissue distribution categories (Figure 8) shows that centrosome and centriolar satellite proteins are not more likely to show any particular type of tissue distribution, compared to all genes presented in the Cell Atlas.
Figure 5. Bar plot showing the percentage of genes in different tissue distribution categories for genes encoding proteins that localize to the centrosome or centriolar satellites, compared to all genes in the Cell Atlas. Asterisk marks a statistically significant deviation (p≤0.05) in the number of genes in a category based on a binomial statistical test. Each bar is clickable and gives a search result of proteins that belong to the selected category.
Relevant links and publications
Thul PJ et al., A subcellular map of the human proteome. Science. (2017)
Bovery T. 1900. Zellen-Studien. Verlag von Gustav Fischer.
Conduit PT et al., Centrosome function and assembly in animal cells. Nat Rev Mol Cell Biol. (2015)