8.A.34 The Endophilin (Endophilin) Family Endophilins A1 and A2 play a role in synaptic vesicle endocytosis in mammals which is regulated by Ca²⁺ influx into neurons (Ren et al., 2006). Both endophilin A1 and A2 exhibit direct binding to voltage-gated Ca²⁺ channels (VGCCs). This interaction is regulated by Ca²⁺ binding to a site in the central domain of endophilins A1 and A2. The endophilin A1 and A2 SH3 domains interact with a proline-rich motif in the central domain adjacent to the Ca²⁺-binding site. Ca²⁺ binding to the central domain of endophilins A1 and A2 breaks this intramolecular SH3 interaction and allows interaction of the central domain with VGCCs. Endophilin A2 association with VGCCs is important in vivo since transient expression in hippocampal neurons of a mutant endophilin A2 construct that constitutively binds VGCCs inhibits endocytosis (possibly by preventing Ca²⁺ influx) (Chen et al., 2003). This regulation of Ca²⁺ influx is somewhat reminiscent of the role of Rvs161p in low-affinity Ca²⁺ influx (TC# 9.A.46) during mating in yeast (Muller et al., 2003). Excitation-contraction (EC) coupling in skeletal muscle requires functional and mechanical coupling between L-type voltage-gated calcium channels (CaV1.1) and the ryanodine receptor (RyR1). STAC3 is an essential protein for EC coupling and is part of a group of three proteins that can bind and modulate L-type voltage-gated calcium channels. Wong King Yuen et al. 2017 reporte crystal structures of tandem-SH3 domains of different STAC isoforms up to 1.2-A resolution. These form a rigid interaction through a conserved interdomain interface. They identified linker connecting transmembrane repeats II and III in two different CaV isoforms as a binding site for the SH3 domains and reported a crystal structure of the complex with the STAC2 isoform. The interaction site includes the location for a disease variant in STAC3 that has been linked to Native American myopathy (NAM). Introducing the mutation does not cause misfolding of the SH3 domains, but abolishes the interaction. Disruption of the interaction via mutations in the II-III loop perturbs skeletal muscle EC coupling, but preserves the ability of STAC3 to slow down inactivation of CaV1.2 (Wong King Yuen et al. 2017).
References:
Chen, Y., L. Deng, Y. Maeno-Hikichi, M. Lai, S. Chang, G. Chen, and J.F. Zhang. (2003). Formation of an endophilin-Ca²⁺ channel complex is critical for clathrin-mediated synaptic vesicle endocytosis. Cell. 115: 37-48.
Friesen, H., K. Colwill, K. Robertson, O. Schub, and B. Andrews. (2005). Interaction of the Saccharomyces cerevisiae cortical actin patch protein Rvs167p with proteins involved in ER to Golgi vesicle trafficking. Genetics 170: 555-568.
Guile, M.D., A. Jain, K.A. Anderson, and C.F. Clarke. (2023). New Insights on the Uptake and Trafficking of Coenzyme Q. Antioxidants (Basel) 12:.
Kirsch, K.H., M.M. Georgescu, S. Ishimaru, and H. Hanafusa. (1999). CMS: an adapter molecule involved in cytoskeletal rearrangements. Proc. Natl. Acad. Sci. USA 96: 6211-6216.
Muller, E.M., N.A. Mackin, S.E. Erdman, and K.W. Cunningham. (2003). Fig1p facilitates Ca2+ influx and cell fusion during mating of Saccharomyces cerevisiae. J. Biol. Chem. 278: 38461-38469.
Mulukala, S.K.N., S.S. Irukuvajjula, K. Kumar, K. Garai, P. Venkatesu, R. Vadrevu, and A.K. Pasupulati. (2020). Structural features and oligomeric nature of human podocin domain. Biochem Biophys Rep 23: 100774.
Ren, G., P. Vajjhala, J.S. Lee, B. Winsor, and A.L. Munn. (2006). The BAR domain proteins: molding membranes in fission, fusion, and phagy. Microbiol. Mol. Biol. Rev. 70: 37-120.
Suthar, S.K., M.M. Alam, J. Lee, J. Monga, A. Joseph, and S.Y. Lee. (2021). Bioinformatic Analyses of Canonical Pathways of TSPOAP1 and its Roles in Human Diseases. Front Mol Biosci 8: 667947.
Suzuki, H., J. Kawai, C. Taga, T. Yaoi, A. Hara, K. Hirose, Y. Hayashizaki, and S. Watanabe. (1996). Stac, a novel neuron-specific protein with cysteine-rich and SH3 domains. Biochem. Biophys. Res. Commun. 229: 902-909.
Takahashi, Y., N. Tsotakos, Y. Liu, M.M. Young, J. Serfass, Z. Tang, T. Abraham, and H.G. Wang. (2016). The Bif-1-Dynamin 2 membrane fission machinery regulates Atg9-containing vesicle generation at the Rab11-positive reservoirs. Oncotarget 7: 20855-20868.
Wong King Yuen, S.M., M. Campiglio, C.C. Tung, B.E. Flucher, and F. Van Petegem. (2017). Structural insights into binding of STAC proteins to voltage-gated calcium channels. Proc. Natl. Acad. Sci. USA 114: E9520-E9528.
Examples:
TC#	Name	Organismal Type	Example
8.A.34.1.1	Endophilin A1 (BAR-domain, SH3-domain containing GRB2-like protein 2 (Endophilin 1 or A1; SH3-domain protein 2A))	Animals	*Endophilin A1 of Mus musculus* (Q62420)**

8.A.34.1.2	*The neuronal regulatory Src homology 3 (SH3) and cysteine-rich domain-containing protein, STAC, of 402 aas and 0 TMSs (Suzuki et al.* 1996).**		STAC of Homo sapiens

8.A.34.1.3	SH3 and cysteine-rich domain-containing protein 3, STAC3 of 364 aas. Excitation-contraction (EC) coupling in skeletal muscle requires functional and mechanical coupling between L-type voltage-gated calcium channels (CaV1.1) and the ryanodine receptor (RyR1), and STAC3 is an essential protein for EC coupling. It is part of a group of three proteins that can bind and modulate L-type voltage-gated calcium channels (Wong King Yuen et al. 2017).		STAC3 of Homo sapiens

8.A.34.1.4	Endophilin-B1 of 365 aas; also called Bif-1. Acts with dynamin 2 (DNM2; P50570; similar to 9.A.63.1.1) to cause membrane fission of vesicles/tubular structures containing the Atg9 protein of 839 aas and 6 - 8 TMSs (9.a.15.2.1; Takahashi et al. 2016).		Endophilin-B1 of Homo sapiens

8.A.34.1.5	CD2-associated protein, CD2AP, of 639 aas. It acts as an adapter protein between membrane proteins and the actin cytoskeleton (Kirsch et al. 1999). Several proteins including podocin (TC# 8.A.21.1.2), nephrin (8.A.23.1.33), CD2AP, and TRPC6 (1.A.4.1.5) form a macromolecular assembly that constitutes the "slit-diaphragm" that functions like a tight junction in podocytes (Mulukala et al. 2020).		CD2AP of Homo sapiens

Examples:
TC#	Name	Organismal Type	Example
8.A.34.2.1	Reduced viability upon starvation protein, Rvs161 (also called Amphiphysen-like lipid raft protein, BAR domain protein, and BAR adaptor protein). (Muller et al, 2003; Ren et al., 2006). It may play a role in the uptake and trafficing of coenzyme Q (Guile et al. 2023).	Fungi	*Rvs161 of Saccharomyces cerevisiae* (P25343)**

8.A.34.2.2	Reduced viability upon starvation protein, RVS167, of 482 aas and possibly 3 or 4 TMSs. It is a component of a cytoskeletal structure that is required for the formation of endocytic vesicles at the plasma membrane level. It has been implicated in cytoskeletal reorganization in response to environmental stresses and could act in the budding site selection mechanism (Friesen et al. 2005). It may play a role in the uptake and trafficking of Coenzyme Q (Guile et al. 2023).		RVS167 of Saccharomyces cerevisiae

Examples:
TC#	Name	Organismal Type	Example
8.A.34.3.1	TSPO accessory protein, TSPOAP, of 1857 aas and possibly 0 TMSs. It may have 2 TMSs centered near residue 1010. It is a cytoplasmic protein associated with its mitochondrial transmembrane protein partner translocator protein (TSPO; TC# 9.A.24) (Suthar et al. 2021). TSPO and TSPOAP1 interact via voltage-dependent anion-selective channels (VDAC1/2/3). A heat map analysis indicated that TSPOAP1 has critical roles in inflammatory, neuroinflammatory, psychiatric, and metabolic diseases and disorders, as well as cancer, and it interacts with several other proteins in the cell as well (Suthar et al. 2021).		TSPOAP1 of Homo sapiens