Microtubules are physically robust polymers made up of α/β-Tubulin dimers called protofilaments assembled into groups of 13 around a hollow core about 25 nm in diameter. When these dimer complexes polymerize, they form linear microtubule filaments that display a polar structure with two growing ends. One fast-growing plus-end with exposed β-subunits and one slow-growing minus-end with exposed α-subunits. Microtubules display a highly dynamic structure due to their ability to polymerize or depolymerize rapidly from end-to-end, two counteracting processes that are both regulated by the binding of GTP. The ability of microtubules to maintain highly dynamic polymerization patterns is vital for the cell's ability to adapt its spatial arrangements in response to different environmental conditions, and to execute mechanical work. A characteristic feature of microtubules is that they never reach a steady-state length and thus remain constantly in the process of elongation (polymerization) or shrinkage (depolymerization), a phenomenon known as dynamic instability (Desai A et al, 1997; Conde C et al, 2009). In the Cell Atlas, several substructures of the microtubule cytoskeleton are classified; the cytokinetic bridge, microtubule ends, midbody, midbody ring and the mitotic spindle (Figure 3).

Microtubules are subjected to a number of different post-translational modifications that influence the structure in order to meet the requirements for their different functions, for example acetylation of lysine residues, detyrosination, glycylation and glutamylation (Janke C, 2014; Wloga D et al, 2010). A selection of proteins suitable to be used as markers for microtubules and the substructures are listed in Table 1.

Table 1. Selection of proteins suitable as markers for the microtubules structure or its substructures.

The function of the microtubules

One major function of the microtubule cytoskeleton is to supply mechanical strength to the cytoplasm and maintain the intracellular organization of the organelles. Microtubules are vital for cell migration and for the mobility of organelles within the cell. They make up eukaryotic cilia and flagella, and act as substrates for motor proteins that can move along the filaments to convert chemical energy into mechanical energy, which facilitates movement. Dynein and kinesin are the two largest families of motor proteins. The dyneins move in direction towards the microtubule organizing center (MTOC) whereas the kinesins are directed away from the MTOC. As an important platform for organelle movement, the microtubule skeleton is a key member of the secretory pathway, where it channels the post-Golgi vesicles out to the plasma membrane. This is done by the help of motor proteins that attach to the vesicles (Schmoranzer J et al, 2003). Another highly important and well studied function of the microtubules is to participate in cell division. Here, they form the mitotic spindle (see Figure 3) in order to properly segregate the chromosomes during mitosis, a function with significant implications in cancer where the mitotic rate is out of control. During the last steps of mitosis, a protein structure called the midbody (see Figure 3) is compacted between the two daughter cells, held together by the cytokinetic bridge (see Figure 3) (Skop AR et al, 2004). Several diseases are linked to defective cellular transport due to abnormalities in the microtubules. Many efficient anti-cancer drugs therefore target the microtubules (Jordan MA et al, 2004). Hereditary diseases associated with defects in cilia, known as ciliopathies, and several neurodegenerative disorders such as Parkinson's syndrome are two of such diseases (Waters AM et al, 2011; Cappelletti G et al, 2015). Gene Ontology (GO)-based analysis of the corresponding genes in the microtubule proteome shows functions highly in line with existing literature. The most highly enriched terms for the GO domain Biological Process are related to microtubule-based processes such as cytoskeleton organization, cilium morphogenesis and cell division (Figure 4a). Enrichment analysis of the GO domain Molecular Function also shows top hits for enriched terms related to motor activity and tubulin binding (Figure 4b).

Table 2. Highly expressed microtubules proteins across different cell lines.

Microtubules proteins with multiple locations

Approximately 71% (n=263) of the microtubule-localizing proteins detected in the Cell Atlas also localize to other compartments in the cell (Figure 5). The network plot shows that the most common locations shared with microtubules are the nucleoplasm, the cytosol and vesicles. As the microtubules are essential for intracellular transport, often utilized by molecules encapsulated in vesicles, the dual locations of these proteins reflect the important role of the microtubules as a transport system in the cell.

Expression levels of microtubules proteins in tissue

Transcriptome analysis (Figure 6) shows that microtubule-localizing proteins are less likely to be expressed in all tissues, compared to all genes with protein data in the Cell Atlas.

Relevant links and publications

Cappelletti G et al, 2015. Linking microtubules to Parkinson's disease: the case of parkin. Biochem Soc Trans.
PubMed: 25849932 DOI: 10.1042/BST20150007

Conde C et al, 2009. Microtubule assembly, organization and dynamics in axons and dendrites. Nat Rev Neurosci.
PubMed: 19377501 DOI: 10.1038/nrn2631

Desai A et al, 1997. Microtubule polymerization dynamics. Annu Rev Cell Dev Biol.
PubMed: 9442869 DOI: 10.1146/annurev.cellbio.13.1.83

Janke C. 2014. The tubulin code: molecular components, readout mechanisms, and functions. J Cell Biol.
PubMed: 25135932 DOI: 10.1083/jcb.201406055

Jordan MA et al, 2004. Microtubules as a target for anticancer drugs. Nat Rev Cancer.
PubMed: 15057285 DOI: 10.1038/nrc1317

Schmoranzer J et al, 2003. Role of microtubules in fusion of post-Golgi vesicles to the plasma membrane. Mol Biol Cell.
PubMed: 12686609 DOI: 10.1091/mbc.E02-08-0500

Skop AR et al, 2004. Dissection of the mammalian midbody proteome reveals conserved cytokinesis mechanisms. Science.
PubMed: 15166316 DOI: 10.1126/science.1097931

Waters AM et al, 2011. Ciliopathies: an expanding disease spectrum. Pediatr Nephrol.
PubMed: 21210154 DOI: 10.1007/s00467-010-1731-7

Wloga D et al, 2010. Post-translational modifications of microtubules. J Cell Sci.
PubMed: 20930140 DOI: 10.1242/jcs.063727