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Summary Background Naturally occurring variants with deletions or mutations in the C‐terminal PreS1 domain from hepatitis B virus (HBV) chronically infected patients have been shown to promote HBsAg retention, inhibit HBsAg secretion and change the extracellular appearance of PreS1‐containing HBV particles (filaments and virions). Aims To study the impact of N‐terminal deletion in preS1 domain on viral secretion and morphogenesis. Methods An HBV mutant with 15 amino acids (aa 25‐39) deletion in N‐terminal preS1 was isolated. Intracellular and extracellular HBsAg were quantified by Western blot. Subcellular HBsAg distribution was analysed by confocal laser scanning microscopy. The viral morphology was characterised by sucrose density gradient ultracentrifugation, Western blot, electron microscopy, HBV mixed ELISA and HBV particle gel essay. Results Expression of this mutant genome released higher amounts of HBsAg in the form of shorter filaments. A significant fraction of semi‐enveloped virions was observed in the supernatant that has been unprecedented so far. Stepwise insertion of aa 25‐31, aa 32‐39 and aa 25‐39 increased the length of filaments. The rescue of aa 25‐31 and aa 25‐39 drastically reduced the amounts of extracellular HBsAg and semi‐enveloped virions, while such effects could not be observed after insertion of aa 32‐39, arguing against a simple spacer function of this region. The deletion and rescued mutants do not differ in subcellular HBsAg distribution and colocalisation with ER, Golgi and multivesicular bodies markers arguing against differences in release pathways. Conclusion N‐terminal PreS1‐domain (aa 25‐31) determines HBsAg secretion and triggers proper assembly of PreS1‐containing particles.

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