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Phosphorylation of breast-milk αS1-casein induced conformational changes and abolished TLR4-agonisticity as well as formation of fibril structure
* 1 , 1 , 2 , 1 , 3 , 3 , 3 , 1
1  Westfälische Wilhelms-Universität, Institut für Pharmazeutische und Medizinische Chemie, PharmaCampus, Correnstr. 48, 48149 Münster, Germany.
2  Westfälische Wilhelms-Universität, Institut für Pharmazeutische Biologie und Phytochemie, PharmaCampus, Correnstr. 48, 48149 Münster, Germany.
3  Heinrich-Heine-Universität Düsseldorf, Universitätsklinikum, Poliklinik für Rheumatologie und Hiller Forschungszentrum Rheumatologie, Moorenstr. 5, 40225 Düsseldorf, Germany.

Published: 30 October 2019 by MDPI in 5th International Electronic Conference on Medicinal Chemistry session ECMC-5

Breast-milk αS1-casein is a Toll-like receptor (TLR4) agonist which induced proinflammatory cytokine secretion. Phosphorylated αS1-casein (P- αS1-casein) is non-agonistic.1,2 The objective of this study was to analyze structural characteristics underlying these observations.

Recombinant αS1-casein was shown to exist in two conformations, an α-helical TLR4-agonistic conformation and a non-agonistic conformation with lower α‑helical and higher random coil content. TLR4-agonstic αS1-casein conformation was found at a pH-range between 7.4 and 2. αS1-Casein bound itself (KD-value: 2 µM) formed large aggregates (between Ø 73 nm [pH7] and Ø 826.2 nm [pH2]). Using Thioflavin T assay and atomic force microscopy showed that αS1-casein adopted fibril-like structure. P-αS1-casein was observed in a less α‑helical conformation, not inducing IL-8 secretion. P-αS1-casein bound itself stronger (KD-value: 0.5 µM) than αS1-casein and did not form fibrils.

In conclusion, TLR4-agonistic and non-agonistic conformations of αS1-casein could be differentiated. It was demonstrated that human caseins are able to adopt fibril structure. These kind of structures are often disease related. We postulate, that phosphorylation could be a switch of two conformations regulating immunomodulatory effects of human αS1-casein especially in immune system development.

  1. Vordenbaumen, S. et al. Human casein alpha s1 induces proinflammatory cytokine expression in monocytic cells by TLR4 signaling. Mol Nutr Food Res 60, 1079-89 (2016).
  2. Saenger, T. et al. Human αS1-casein induces IL-8 secretion by binding to the ecto-domain of the TLR4/MD2 receptor complex. Biochim Biophys Acta Gen Subj 1863, 632-643 (2019).
Keywords: Breast milk; human αS1-casein; TLR4 agonist; fibril structure, CK2