9.A.35 The Peptide Translocating Syndecan (Syndecan) Family

Syndecans are transmembrane heparan sulfate proteoglycans. They are implicated in the binding of extracellular matrix components and growth factors. Syndecan is made as a precursor with a hydrophobic N-terminal leader peptide and a strongly hydrophobic C-terminal TMS which presumably anchors the protein in the membrane.  Syndecan-4 and beta1 integrin protein levels and their localization in costameric structures are regulated by electrical activity by a mechanism that influences the adhesion properties of skeletal myotubes during differentiation (Ugarte et al. 2010). Multiple dimerizing motifs are present at different locations in the protein, including in the transmembrane domain, and these modulate the homo- and hetero-dimerization of syndecan (Chen et al. 2021). The conformations, interactions and functions of intrinsically disordered external domains of syndecans have been reviewed (Ricard-Blum and Couchman 2023). Syndecans link the cytoskeleton to calcium channels of the transient receptor potential class (TC# 1.A.4), compatible with roles as mechanosensors. In turn, syndecans influence actin cytoskeletal organization to impact ion transport, motility, adhesion and the extracellular matrix environment (Ricard-Blum and Couchman 2023).

Cell-penetrating peptides (CPPs) are short peptides capable of translocating across the plasma membrane of live cells and transporting conjugated compounds intracellularly. The first model cationic CPPs to be discovered were penetratin and TAT. CPPs may enter cells by mediation using a surface receptor. Letoha et al. (2010) reported that syndecan-4, the universally expressed isoform of the syndecan family of transmembrane proteoglycans, binds and mediates transport of the three most frequently utilized cationic CPPs (penetratin, octaarginine and TAT) into the cells. Quantitative uptake studies and mutational analyses demonstrate that attachment of the cationic CPPs is mediated by specific interactions between the heparan sulfate chains of syndecan-4 and the CPPs. Protein kinase C alpha is also involved in uptake. The data presented by Letoha et al. (2010) provide direct evidence for the receptor-mediated uptake of cationic CPPs. ESCRT-to-membrane coupling via ALIX/syntenin/syndecan-4 is required for completion of cytokinesis (Addi et al. 2020).

Syndecans, the evolutionarily conserved family of transmembrane proteoglycans, facilitate the cellular entry of SARS-CoV-2 (Hudák et al. 2021). Among syndecans, the lung abundant syndecan-4 is the most efficient in mediating SARS-CoV-2 uptake. The S1 subunit of the SARS-CoV-2 spike protein plays a dominant role in the virus's interactions with syndecans. Besides the polyanionic heparan sulfate chains, other parts of the syndecan ectodomain, such as the cell-binding domain, also contribute to the interaction with SARS-CoV-2. During virus internalization, syndecans colocalize with ACE2, suggesting a jointly shared internalization pathway. Both ACE2 and syndecan inhibitors exhibit significant efficacy in reducing the cellular entry of SARS-CoV-2, thus supporting the complex nature of internalization (Hudák et al. 2021).



Addi, C., A. Presle, S. Frémont, F. Cuvelier, M. Rocancourt, F. Milin, S. Schmutz, J. Chamot-Rooke, T. Douché, M. Duchateau, Q. Giai Gianetto, A. Salles, H. Ménager, M. Matondo, P. Zimmermann, N. Gupta-Rossi, and A. Echard. (2020). The Flemmingsome reveals an ESCRT-to-membrane coupling via ALIX/syntenin/syndecan-4 required for completion of cytokinesis. Nat Commun 11: 1941.

Baietti, M.F., Z. Zhang, E. Mortier, A. Melchior, G. Degeest, A. Geeraerts, Y. Ivarsson, F. Depoortere, C. Coomans, E. Vermeiren, P. Zimmermann, and G. David. (2012). Syndecan-syntenin-ALIX regulates the biogenesis of exosomes. Nat. Cell Biol. 14: 677-685.

Chen, J., F. Wang, C. He, and S.Z. Luo. (2021). Multiple dimerizing motifs at different locations modulate the dimerization of the syndecan transmembrane domains. J Mol Graph Model 106: 107938.

Gopal, S., A. Amran, A. Elton, L. Ng, and R. Pocock. (2021). A somatic proteoglycan controls Notch-directed germ cell fate. Nat Commun 12: 6708.

Guo, S., X. Wu, T. Lei, R. Zhong, Y. Wang, L. Zhang, Q. Zhao, Y. Huang, Y. Shi, and L. Wu. (2021). The Role and Therapeutic Value of Syndecan-1 in Cancer Metastasis and Drug Resistance. Front Cell Dev Biol 9: 784983.

Hudák, A., A. Letoha, L. Szilák, and T. Letoha. (2021). Contribution of Syndecans to the Cellular Entry of SARS-CoV-2. Int J Mol Sci 22:.

Letoha T., Keller-Pinter A., Kusz E., Kolozsi C., Bozso Z., Toth G., Vizler C., Olah Z. and Szilak L. (2010). Cell-penetrating peptide exploited syndecans. Biochim Biophys Acta. 1798(12):2258-65.

Ricard-Blum, S. and J.R. Couchman. (2023). Conformations, interactions and functions of intrinsically disordered syndecans. Biochem Soc Trans 51: 1083-1096.

Stepp, M.A., S. Pal-Ghosh, G. Tadvalkar, and A. Pajoohesh-Ganji. (2015). Syndecan-1 and Its Expanding List of Contacts. Adv Wound Care (New Rochelle) 4: 235-249.

Sundberg, E.L., Y. Deng, and C.G. Burd. (2019). Syndecan-1 Mediates Sorting of Soluble Lipoprotein Lipase with Sphingomyelin-Rich Membrane in the Golgi Apparatus. Dev Cell. [Epub: Ahead of Print]

Ugarte, G., C. Santander, and E. Brandan. (2010). Syndecan-4 and beta1 integrin are regulated by electrical activity in skeletal muscle: Implications for cell adhesion. Matrix Biol 29: 383-392.


TC#NameOrganismal TypeExample

Syndecan-4 of 198 aas.  Cell surface proteoglycan that bears heparan sulfate. Regulates exosome biogenesis in concert with SDCBP and PDCD6IP. The syndecan-syntenin-ALIX complex plays a role  in membrane transport and signalling processes (Baietti et al. 2012). ESCRT-to-membrane coupling via ALIX/syntenin/syndecan-4 is required for completion of cytokinesis (Addi et al. 2020). Plays a role in SARS-CoV2 entry (see family description) (Hudák et al. 2021).


Syndecan-4 of Homo sapiens (P31431)


Syndecan-1 of 310 aas and 2 TMSs, N- and C-terminal.  The binding of cytokines and growth factors to heparan sulfate (HS) chains on proteoglycans generates gradients that control development and regulate wound healing. Syndecan-1 (sdc1) is an integral membrane HS proteoglycan. Its structure allows it to bind with cytosolic, transmembrane, and extracellular matrix (ECM) proteins. It plays important roles in mediating key events during wound healing because it regulates a number of important processes, including cell adhesion, cell migration, endocytosis, exosome formation, and fibrosis (Stepp et al. 2015). Syndecan-1 mediates sorting of soluble lipoprotein lipase into sphingomyelin-rich membranes in the golgi apparatus (Sundberg et al. 2019). The structural characteristics, shedding process, and nuclear translocation of SDC-1 have been reviewed (Guo et al. 2021).


Syndecan-1 of Homo sapiens


Uncharacterized protein of 220 aas and 2 TMSs.

UP of Coptotermes formosanus (Formosan subterranean termite)


Syndecan-3 of 346 aas and 1 TMS, near the C-terminus.

Syndecan-3 of Globicephala melas (long-finned pilot whale)


Uncharacterized protein of 488 aas and 1 TMS

UP of Trichogramma brassicae


Uncharacterized protein of 248 aas and 1 TMS.

UP of Didymodactylos carnosus


Syndecan-1, SDN-1, of 287 aas and 1 C-terminal TMS. SDN-1 controls a somatic TRP calcium channel and governs calcium-dependent binding of an AP-2 transcription factor (APTF-2) to the glp-1 promoter. SDN-1 signaling promotes GLP-1 expression and mitotic germ cell fate. Thus, SDN-1 is a putative communication nexus between the germline and its somatic environment to control germ cell fate decisions (Gopal et al. 2021).

SDN-1 of Caenorhabditis elegans