Myristoylated peptides are candidates for drug design!

Myristoylation of proteins or peptides is a posttranslational modification found in eukaryotic and viral proteins in which the N-terminal end is acylated with myristic acid. Myristic acid, or Tetradecanoic acid, is a fatty acid with the molecular formula CH₃(CH₃)₁₂COOH, and a molecular weight of 228.27 g/mol. The modification of the protein N-terminus with a myristoyl group [a (cis,cis-delta 5, delta 8)-tetradecadienoyl group =  myristoylation with 2 double bonds] adds an average mass of 206 dalton, and the myristoleylation (= myristoyl with one double bond) adds a mass of 208 daltons to the target protein. Modern peptide synthesis chemistry now allows for the production of modified peptides such as myristoylated or palmityolated peptides.

N-myristoylation refers to the acylation process specific to the N-terminal amino acid glycine in proteins. This process has been observed in hundreds of proteins in lower and higher eukaryotes. Proteins labeled with this modification are involved in oncogenesis, in secondary cellular signaling, in infectivity of retroviruses and other virus types. The cytosolic enzyme responsible for this modification is N-myristoyltransferase (NMT).

If NMT is selectively inhibited a dose-responsive loss of N-myristoylation on protein targets is observed. A recent proteomic study showed that NMT inhibition killed HeLA cells apparently through endoplasmic reticulum (ER) stress and the unfolding response signalling (UPR) pathways. 

Also, Rampoldi et al. in 2015 found that protein myristoylation is indispensable in T cell development and activation. A deficiency in N-myristoyl transferase (Nmt) 1 and 2 activity caused a defective transmission of TCR signals, a developmental blockage of thymocytes at the transition from double-negative 3 to 4 stages, and a reduction of all following stages. Further more, the study found that two main myristoylated kinases in T cells were mislocalized in the absence of Nmt activity, and the absence of myristoylation resulted in an immunosuppressive effect on T cells.

Figure 1:  Crystal structure based model of calcium-calmodulin (Ca2+-CaM) bound to a myristoylated peptide derived from the N-terminal domain of CAP-23/NAP-22.

Source: https://pixabay.com/en/hepatitis-b-virus-virus-3d-1186581/

König et al. in 2014 showed that N-terminal myristoylated lipopeptides, derived from the pre-S protein of the hepatitis B virus, can be used to study their binding kinetics to the human liver bile acid transporter Na+/taurocholate cotransporter. The ultimate goal of this study is the design of lipopeptide-based drugs useful for the management of hepatitis B infections.

According to König et al., the binding of myristoylated hepatitis B virus preS1 (myr-preS1) protein to the human liver bile acid transporter Na+/taurocholate cotransporter (NTCP) is necessary for a productive infection by the hepatitis B virus (HBV). The infection interferes with the physiological bile acid transport function of NTCP. The binding of the myr-preS1 peptide to NTCP initiates the infection. König et al. investigated the binding kinetics of myr-preS1 peptides to NTCP. The results of the study suggested that the myr-preS1 peptide inhibits bile acid transport in the cells. Cell lines used for the studies where primary Tupaia belangeri (PTH; northern treeshrew; an animal model) and human (PHH) hepatocytes as well as NTCP-transfected human hepatome HepG2 cells. The research group suggests that NTCP substrates may be useful tools for the design of NTCP-inhibiting drugs allowing effective management of HBV infections. 

Plants, more specifically maize, have been investigated to allow high level expression of Hepatitis B Surface Antigens for the development of plant-based oral HBV vaccines.

The myristoylated peptide used for the study was the following peptide: 

Myr-GQNLSTSNPLGFFPDHQLDPAFRANTANPDWDFNPNKDTWPDANKVG-C(Alexa594)-COOH

Many viral and signal transduction proteins are known to be myristoylated. The N-terminal myristoyl group is directly involved in protein-protein interactions. For some proteins, this modification appears to be essential for proper functioning of the modified proteins. For example, the activity of p60src from Rous sarcoma virus is dependent on its myristoylation. It is assumed that hydrophobic acyl groups, for example, myristoyl and palmitoyl groups, are often involved in protein-protein interactions.

The crystal structure of calcium-calmodulin (Ca2+-CaM) bound to a myristoylated peptide derived from the N-terminal domain of CAP-23/NAP-22 has been solved see figure 1). Cap-23/NAP-22 is a brain-specific protein kinase C substrate involved in axon regeneration that binds calmodulin with high affinity. A synthetic myristoylated peptide of nine amino acids and purified recombinant human CaM was used for hanging-drop vapor diffusion crystallization. The researchers were able to solve the structure of the complex from a single crystal at a resolution of 2.3 Å.

Reference

Boutin JA.; Myristoylation. Cell Signal. 1997 Jan;9(1):15-35.

P. GRIPON, J. LE SEYEC, S. RUMIN, C. GUGUEN-GUILLOUZO; Myristylation of the Hepatitis B Virus Large Surface Protein Is Essential for Viral Infectivity. Virology. 1995 Nov 10;213(2):292-9.

König, Alexander et al.; Kinetics of the bile acid transporter and hepatitis B virus receptor Na+/taurocholate cotransporting polypeptide (NTCP) in hepatocytes. Journal of Hepatology , Volume 61 , Issue 4 , 867 – 875.  https://www.ncbi.nlm.nih.gov/pubmed/24845614.

Matsubara M, Nakatsu T, Kato H, Taniguchi H.; Crystal structure of a myristoylated cap-23/nap-22 n-terminal domain complexed with ca2+/calmodulin. Embo J. (2004) 23 p.712.

Francesca Rampoldi, Mahnaz Bonrouhi, Martin E. Boehm, Wolf D. Lehmann, Zoran V. Popovic, Sylvia Kaden, Giuseppina Federico, Fabian Brunk, Hermann-Josef Gröne and Stefan Porubsky; Immunosuppression and Aberrant T Cell Development in the Absence of N-Myristoylation. The Journal of Immunology November 1, 2015 vol. 195 no. 9 4228-4243.

Thinon E, Morales-Sanfrutos J, Mann DJ, Tate EW.; N-Myristoyltransferase Inhibition Induces ER-Stress, Cell Cycle Arrest, and Apoptosis in Cancer Cells. ACS Chem Biol. 2016 Aug 19;11(8):2165-76. doi: 10.1021/acschembio.6b00371.  PMID: 27267252.

Wang, Miao, Kaufman, Randal J.; The impact of the endoplasmic reticulum protein-folding environment on cancer development. Nat Rev Cancer 14, 581-597 (2014) http://dx.doi.org/10.1038/nrc3800.

Chunyan Xu, Beatrice Bailly-Maitre, and John C. Reed; Endoplasmic reticulum stress: cell life and death decisions. J Clin Invest. 2005 Oct 1; 115(10): 2656–2664.  doi:  10.1172/JCI26373 PMCID: PMC1236697.

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