BBSome

A BBSome is a protein complex that operates in primary cilia biogenesis, homeostasis, and intraflagellar transport (IFT).Klink, B.U., Gatsogiannis, C., Hofnagel, O., Wittinghofer, A., and Raunser, S.(Jan 2020). Structure of the human BBSome core complex. Electron Microscopy Data Bank. doi: 10.7554/eLife.53910 The BBSome recognizes cargo proteins and signaling molecules like G-protein coupled receptors (GPCRs) on the ciliary membrane and helps transport them to and from the primary cilia.Klink, B.U., Zent, E., Juneja, P., Kuhlee, A., Rausner, S., and Wittinghofer, A. (Nov 2017). A recombinant BBSome core complex and how it interacts with ciliary cargo. eLife". doi: 10.7554/eLife.27434 Lotkev, A.V., Zhang, Q., Beck, J.S., Searby, C.C., Scheetz, T.E., Bazan, J.F., Slusarski, D.C., Sheffield, V.C., Jackson, P.K., and Nachury, M.V. (Dec 2008). A BBSome subunit links ciliogenesis, microtubule stability, and acetylation. Developmental Cell. 15 (6):854-865 DOI: 10.1016/j.devcel.2008.11.001 Primary cilia are nonmotile microtubule projections that function like antennae and are found in many types of cells.Jin, H. and Nachury, M.V. (June 2009). The BBSome. "Current Biology. 19 (12):472-473. doi.org/10.1016/j.cub.2009.04.015. They receive various environmental signals to aid the cell in survival. They can detect photons by concentrating rhodopsin, a light receptor that converts photons into chemical signals, or odorants by concentrating olfactory receptors on the primary cilia surface.Satir, P. (Mar 2017). CILIA: before and after. Cilia. 6:1–11. doi: 10.1186/s13630-017-0046-8. Primary cilia are also meaningful in cell development and signaling. They do not contain any way to make proteins within the primary cilia, so the BBSome aids in transporting essential proteins to, from, and within the cilia. Examples of cargo proteins that the BBSome is responsible for ferrying include smoothened (a component of the Hedgehog signaling pathway), polycystic-1 (PC1), and several G-Protein coupled receptors (GPCRs) like somatostatin receptors (Sstr3), melanin-concentrating hormone receptor 1 (Mchr1), and neuropeptide Y2 receptor.Scheidecker, S., Etard, C., Pierce, N.W., Geoffroy, V., Schaefer, E., et al. (Feb 2014). Exome sequencing of Bardet-Biedl syndrome patient identifies a null mutation in the BBSome subunit BBIP1 (BBS18). Journal of Medical Genetics. DOI:10.1136/jmedgenet-2013-101785 Zhao, Y. and Rahmouni, K. (Dec 2021). BBSome: a New Player in Hypertension and Other Cardiovascular Risks. Hypertension. 79 (2):303-313 doi.org/10.1161/HYPERTENSIONAHA.121.17946

The BBSome is an eight-protein complex consisting of different subunits named Bardet-Biedl Syndrome (BBS) proteins after the ciliopathy disease caused by a mutation in BBS proteins. Currently, there are 24 discovered BBS gene products that either form the BBSome or interact with the BBSome.Weerakkody, Y. (Jan 2023). Postaxial polydactyly. "Radiopedia. doi.org/10.53347/rID-15588. Several BBS proteins that are not associated with the BBSome (BBS11, BBS13, BBS15, BBS16, BBS19-24) have yet to be extensively studied. The proteins within the largest and most stable BBSome core complex are BBS1, BBS4, BBS5, BBS8, BBS9, and BBS-interacting protein BBIP1, also known as BBS18. BBIP1 is the proposed eighteenth BBS gene due to its essential role in interacting with the BBSome and reduced levels in patients with a BBS diagnosis. BBS2 and BBS7 are also within the BBSome but are more loosely associated, which leads to their exclusion in the core complex. Other BBSome-associated proteins can be found in higher-level organisms with more IFT requirements. For example, BBS6, BBS10, and BB12 form into chaperonin complexes with CCT/TRiC (chaperonin-containing tailless complex polypeptide 1/tailless complex polypeptide 1 ring complex) in chordates which function to regulate and oversee the assembly of the BBSome in an ATP-dependent manner. BBS3 associates with Arl6 (ARF-like 6) and helps in recruiting the BBSome to ciliary membranes.The Human Gene Database. (Mar 2023). ARL6 Gene. "Gene Cards. BBS7 interacts with LZTFL1 (Leucine Zipper Transaction Factor-Like 1) to regulate the entry of the BBSome into the primary cilia. The BBSome has a similar structure and function compared to COPI, COPII, and clathrin coats showing its similarity to the function of these vesicle-forming complexes in transporting proteins.Jin, H., White, S.R., Shida, T., Schulz, S., Aguiar, M., Gygi, S.P., Bazan, J.F., and Nachury, M.V. (Jun 2011). The conserved bardet-biedl syndrome proteins assemble a coat that traffics membrane proteins to cilia. Cell. 141 (7):1208–1219. doi: 10.1016/j.cell.2010.05.015. All BBS proteins are highly conserved in genetics which shows their importance in primary cilium biogenesis and intraflagellar transport (IFT).

The BBSome links cargo proteins to intraflagellar transport (IFT) machinery, which transports structural components and receptors, with the help of motor proteins dynein and kinesin, from the tip to the base of the primary cilia (anterograde transport) and back (retrograde transport) along ciliary microtubules.Wingfield, J.L., Lechtreck, K.F., and Lorentzen, E. (Sep 2019). Trafficking of ciliary membrane proteins by the intraflagellar transport/BBSome machinery. Essays in Biochemistry. 62 (6):753–763. doi: 10.1042/EBC20180030. Since cilia cannot synthesize proteins, the IFT pathway is required for biogenesis, maintenance, and signaling within the cilia through motors, IFT-A and IFT-B subcomplexes, and the cargo proteins.Wei, Q., Zhang, Y., Li, Y., Zhange, Q., Ling, K, and Hu, J. (Aug 2012). The BBSome controls IFT assembly and turnaround in cilia. Natural Cell Biology". 14:950-957 doi-org./10.1038/ncb2560 The BBSome assists with the assembly and stabilization of the IFT complex at the ciliary base and mediates the bidirectional movement, all of which sustains the success of IFT. IFT-A controls retrograde IFT, and IFT-B controls anterograde transport. DYF-2 is a protein that functions with BBS1 to stabilize the interaction between the BBSome and the IFT complex in preparation for retrograde transport. In mutants with nonfunctional BBSome proteins, IFT-B can not associate with IFT-A, which demonstrates the BBSome function of assembling the IFT machinery. An experiment performed with Caenorhabditis elegans looked at GFP-tagged IFT-B protein complexes to look for IFT-B accumulation at the tip of the primary cilia in organisms with inhibited IFT turnaround. The defective (Dyf) C. elegans mutants showed a dissociation between the BBSome and IFT particles causing BBS proteins to accumulate at the ciliary base, regular anterograde transport, but an accumulation of IFT-B components at the ciliary tip due to an absence of the BBSome.

BBS gene expression has been observed in nonciliated cells in cardiac, vascular, and renal tissues, which expands the parameters of the BBSome functions to cellular processes other than solely primary cilia protein transport, such as plasma membrane receptor localization, gene expression, and cell division. The discovery that BBS7, and other BBS proteins, like BBS4, enter the nucleus and, in the case of BBS7, interact with ring finger protein 2 (RNF2) to regulate its transcription supports the concept that the BBSome might also be involved in gene expression.Gascue, C., Tan, P.L., Cardenas-Rodriguez, M., Libisch, G., Fernandez-Calero, T., Liu, Y.P., Astrada, S., Robello, C., Naya, H., Katsanis, N., and Badano, J.L. (Jan 2012) Direct role of bardet-biedl syndrome proteins in transcriptional regulation. Journal of Cell Science. 125(2):362–375 doi: 10.1242/jcs.089375 It is not yet definitive on whether this gene expression role is separate from the BBSome transportation of proteins function.

Discovery It was discovered by Maxence Nachury, Alexander Loktev, and several other associates in a study performed in 2007 that used biochemical purification of complexes that contained BBS4 in mammalian cells. The researchers identified BBSome localization to both centriolar satellites in the cytoplasm and the membrane of the cilia, the importance of the BBSome concerning IFT, and the mechanism for delivery to the cilia via centriolar satellites. They hypothesized that the BBSome was transported to the basal body by centriolar satellites, which are cytoplasmic granules that bring specific proteins to the centrosome due to its connection to PCM-1. They proposed the use of centriolar satellites to transport the BBSome was to function in a chaperone-like manner to limit BBSome activity to the basal body. The researchers noticed that BBS4 interacts with PCM-1, a core component of centriolar satellites, and helps bring proteins to the centrosome. Then, the BBSome can dissociate from PMC-1 and perform its function in the basal body or within the primary cilia. The researchers believed the complex consisted of seven proteins (BBS1, BBS2, BBS4, BSS5, BBS7, BBS8, and BBS9). They also discovered the relationship between the subunits of interactions of BBS1/BBS2/BBS7 with β-propeller domains, BBS4/BBS8 with TPR (tetratricopeptide repeat) regions, BBS3/Arl6, and BBS6/BBS10/BBS12 relations for chaperonin-like functions. They noticed that BBS9 interacted with several other subunits like BBS1, BBS2, BBS4, BBS5, and BBS8, so they proposed that it was located in the center of the complex. They also noticed how the BBSome function was linked to Rabin8, a protein that localizes the basal body and attracts the BBSome protein BBS1. The scientists then looked at BBS5 to determine the areas of the BBSome that function in lipid binding and noticed how the binding of phosphoinositides might be essential for ciliogenesis. Arl6's crystal structure allowed for high-resolution structural information on the BBSome subunits to be collected when the BBSome is active (Arl6 bound).Mourão, A., Nager, A.R., Nachury, M.V., and Lorentzen, E. (Jun 2015). Structural basis for membrane targeting of the BBSome by ARL6. Nature Structural & Molecular Biology. 21 (2):1035–1041. doi: 10.1038/nsmb.2920. High-resolution cryo-electron microscopy with an average of 3.8 Å was used to display the overall complex structure, but it did not allow for an accurate atomic model due to limited resolution. Only approximately 80% of the complex could be described using an atomic model. Due to the relatively recent discovery of the protein complex, there is still a lot about the mechanism in which it functions that remain unknown. The identity of which cilia membrane proteins require the BBSome, the molecular and/or enzymatic activity of the complex, the specific function of BBS proteins that are not within the BBSome, as well as many other questions still need to be answered for the complete understanding of what the BBSome is and how it functions.

Structure and organization

Assembly It is believed that the BBSome assembles sequentially, beginning with the association of BBS7, BBS chaperonins, and the CCT/TRiC complex, which functions as a scaffold to which further subunits can bind.Zhang, Q., Yu, D., Seo, S., Stone, E.M., and Sheffield, V.C. (Jun 2012). Intrinsic protein-protein interaction-mediated and chaperonin-assisted sequential assembly of stable bardet-biedl syndrome protein complex, the BBSome. Journal of Biological Chemistry. 287 (24):20625–20635. doi: 10.1074/jbc.M112.341487 BBS2 and BBS7 bind together first. BBS2 and BBS7 are missing in certain species, such as Drosophila, which leads to the idea that they might be more important in organizing the BBSome in higher organisms rather than directly related to the cargo and membrane binding functions of the BBSome. Then, BBS9 is added to the core complex followed by BBS5 and BBS8.

Core structure