Plasma membraneThe plasma membrane, also known as the cell membrane or cytoplasmic membrane, is the barrier that encloses the cell and protects the intracellular components from the surroundings. The plasma membrane is a thin semi-permeable membrane consisting of a lipid bilayer and associated proteins, each constituting about 50% of the total mass. Example images of proteins localized to the plasma membrane can be seen in Figure 1. In the subcellular resource, 2400 genes (12% of all protein-coding human genes) have been shown to encode proteins that localize to the plasma membrane (Figure 2). A Gene Ontology (GO)-based functional enrichment analysis of genes encoding proteins that localize to the plasma membrane shows enrichment of genes associated with biological processes related to structural intracellular organization, cell signalling and cellular response to extracellular stimuli, transport across the plasma membrane, and cell adhesion. Similarly, the enriched GO terms for molcular functions revolve around channel and transporter activity, and binding to proteins associated with the plasma membrane. About 80% of the plasma membrane proteins localize to other cellular compartments in addition to the plasma membrane, with co-localization between the plasma membrane and actin filaments, vesicles or the cytosol being overrepresented.
Figure 1. Examples of proteins localized to the plasma membrane. EGFR is a transmembrane glycoprotein that binds to Epidermal Growth Factor (detected in A-431 cells). CTNNB1 plays a key role in regulation of transcription in response to the Wnt signalling pathway, but also in the formation of adherens junctions as a subunit of the cadherin complex (detected in A-431 cells). EZR plays a key role in cell surface structure adhesion, migration and organization (detected in A-431 cells).
Figure 2. 12% of all human protein-coding genes encode proteins localized to the plasma membrane. Each bar is clickable and gives a search result of proteins that belong to the selected category. The structure of the plasma membraneSubstructures The plasma membrane is composed of a lipid bilayer, in which lipids constitute half and proteins the other half of the total mass in most human cell types. Phospholipids, which are composed of a hydrophilic phosphate group and two hydrophobic fatty-acid chains, make up the fundamental structural element in the plasma membrane (Jacobson K et al. (2019); Kobayashi T et al. (2018),Alberts B et al, 2002b. The inner and outer leaflet of the bilayer is held together by non-covalent interactions between the hydrophobic tails, which point towards each other and away from the hydrophilic faces of the membrane. In addition to phospholipids, the plasma membrane of animal cells contains two other major lipid classes; glycolipids and cholesterol. While cholesterol is usually almost as abundant as phospholipids, glycolipids only constitute about 2% of the lipids of the plasma membrane and are found only in the outer leaflet. The second major component of the plasma membrane is proteins. They can be divided into integral membrane proteins that cross the complete bilayer, peripheral membrane proteins that are anchored into one leaflet of the lipid bilayer, and surface proteins that bind to the polar heads of phospholipids or other membrane proteins. The composition of the plasma membrane is dynamic and adapts to changes in the environment as well as to the cell cycle. At physiological temperatures, the cell membrane is fluid and flexible, while at cooler temperatures, it becomes gel-like. While the plasma membrane is behaving like a two-dimensional fluid, in which the lipids and proteins are not in fixed positions, it is still organized in different microdomains and specialized regions (Krapf D. (2018); Jacobson K et al. (2019); Kobayashi T et al. (2018)). These include lipid rafts, caveolae, protrusions and cell junctions. Cell junctions (Figure 3) consist of regions with protein complexes that mediate contact or adhesion with neighbouring cells or with the extracellular matrix (Garcia MA et al. (2018)). The major types of cell junctions in vertebrates include gap junctions, tight junctions, and anchoring junctions. The latter includes desmosomes, hemidesmosomes and adherens junctions. Desmosomes mediate cell-cell adhesion through transmembrane linker-proteins called cadherins, which connect to intermediate filaments within the cell and to cadherins on neighbouring cells. Hemi-desmosomes instead contain integrins, which also connect to intermediate filaments in the cytosol, but to components of the extracellular matrix instead of neighbouring cells. Adherens junctions can contain cadherins or integrins, but in this case connects to actin filaments in the cytosol. A selection of proteins suitable to be used as markers for the plasma membrane is listed in Table 1. A list of highly expressed genes encoding proteins that localize to the plasma membrane can be found in Table 2. Table 1. Selection of proteins suitable as markers for the plasma membrane.
Table 2. Highly expressed single localizing plasma membrane proteins across different cell lines.
Figure 3. Examples of proteins localized to different types of cell junctions. CDH17 (Cadherin 17) is a membrane-associated glycoprotein. Cadherins are calcium dependent cell adhesion proteins (detected in CACO-2 cells). GJB6 is a gap junction protein through which small materials diffuse into neighboring cells (detected in RT4 cells). TJP3 plays a role in the linkage between the actin cytoskeleton and tight junctions (detected in CACO-2 cells).
Figure 4. 3D-view of the plasma membrane in U2OS, visualized by immunofluorescent staining of EZR. The morphology of plasma membrane in human induced stem cells can be seen in the Allen Cell Explorer. The function of the plasma membraneThe plasma membrane is involved in a variety of cellular processes (Alberts B et al, 2002b). The main function of the plasma membrane is to separate and protect the intracellular environment from the extracellular space. The plasma membrane is semi-permeable and selectively regulates the passage and transport of various molecules and compounds in and out of the cell. For small molecules, such as ions, cross-membrane cellular transport can occur by passive osmosis and diffusion, but transport against the concentration gradient requires the help of ion pumps. For larger molecules, like hormones and enzymes, transport occurs by endocytosis, exocytosis or with the help of transmembrane protein transporters or channels. The plasma membrane also provides structural integrity, shape and polarity to cells by anchoring the cytoskeleton and by attaching the cell to the extracellular matrix and to other cells (Orlando K et al. (2009)). These physical connections, as well as the presence of receptors or other factors with a role in signal transduction, are also essential for cell-cell and cell-ECM communication. Moreover, the plasma membrane has a central role in cellular motility and polarity (Eaton RC et al. (1991)). A rupture in the plasma membrane leads to the impairment of cell integrity and function, resulting in cell lysis and cell death unless rapidly repaired. Mutations in genes encoding proteins that localize to the plasma membrane have been associated with numerous human diseases. For example, mutations in genes encoding channel- and transporter proteins have been linked to cystic fibrosis, cardiac arrhythmia, diabetes, skeletal muscle defects, and neurological disorders. Moreover, disturbances in the composition of membrane lipids and proteins may lead to a variety of diseases related to lipid metabolism (Simons K et al. (2002)). Gene Ontology (GO)-based functional enrichment analysis of genes encoding proteins localizing to the plasma membrane shows enrichment of terms describing functions that are well in-line with the known functions of the plasma membrane. The most highly enriched terms for the GO domain Biological Process are related to cell adhesion, cell signalling, transport across the plasma membrane, and structural organization of the plasma membrane and related structures (Figure 5a). Enrichment analysis of the GO domain Molecular Function gives top hits for terms related to binding to adhesion molecules and receptors, as well as receptor activity and channel activity (Figure 5b).
Figure 5a. Gene Ontology-based enrichment analysis for the plasma membrane 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 5b. Gene Ontology-based enrichment analysis for the plasma membrane 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. Plasma membrane proteins with multiple locationsApproximately 83% (n=1983) of the plasma membrane proteins detected in the subcellular resource also localize to other cellular compartments (Figure 6). The network plot shows that the most common additional locations for proteins that localize to the plasma membrane are the cytosol, nucleoplasm, vesicles and actin filaments. Proteins that localize to both the plasma membrane and to the cytosol are overrepresented, perhaps reflecting that many proteins, such as adaoptor proteins, move between the cytosol and the inner surface of the plasma membrane as part of signalling pathways. There is also an overrepresentation of proteins that localize to the plasma membrane and to actin filaments. Indeed, actin filaments are often concentrated just beneth the plasma membrane and anchored to the plasma membrane through various actin-binding proteins. This close connection between actin filaments and the plasma membrane determines cell shape and is involved in a variety of cell surface activities. Overrepresentation of proteins that localize to both the plasma membrane and vesicles may reflect the numerous transport vesicles that deliver newly synthesized lipids and proteins to the plasma membrane. Examples of multilocalizing plasma membrane proteins can be seen in Figure 7.
Figure 6. Interactive network plot of the plasma membrane proteins with multiple localizations. The numbers in the connecting nodes show the proteins that are localized to the plasma membrane and to one or more additional locations. Only connecting nodes containing at least 1.0% of the proteins in the plasma membrane 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.
Figure 7. Examples of multilocalizing proteins in the plasma membrane proteome. BAIAP2 is an adapter protein that links membrane bound G-proteins, which plays a role in signal transduction, to cytoplasmic effector proteins. It has been shown to localize to both the cytoplasm and the plasma membrane (detected in U2OS cells). ADD1 is a heterodimeric protein. It binds with high affinity to Calmodulin and is a substrate for protein kinases. It has been shown to localize to both the nucleus and the plasma membrane (detected in Hep-G2 cells). ARHGEF26 is a member of the Rho-guanine nucleotide exchange factor (Rho-GEF). These proteins regulate Rho GTPases by catalyzing the exchange of GDP for GTP. GTPases act as molecular switches in intracellular signaling pathways. It has been shown that ARHGEF26 localizes to the nucleus, cytoplasm and plasma membrane (detected in U-251 MG cells). Expression levels of plasma membrane proteins in tissueTranscriptome analysis and classification of genes into tissue distribution categories (Figure 8) shows that a significantly larger portion of the plasma membrane-associated protein-coding genes are detected in some or in many tissues, while a smaller portion are detected in all tissues, compared to all genes presented in the subcellular resource. This indicates a more pronounced role for plasma membrane proteins in functions or structures specific to groups of tissues. Figure 8. Bar plot showing the percentage of genes in different tissue distribution categories for plasma membrane-associated protein-coding genes, compared to all genes in the subcellular resource. 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 Jacobson K et al., The Lateral Organization and Mobility of Plasma Membrane Components. Cell. (2019) |