The matrix protein M1 of influenza A virus forms a shell beneath the viral envelope and sustains the virion architecture by interacting with other viral components. A structural change of M1 upon acidification of the virion interior in an early stage of virus infection is considered to be a key step to virus uncoating. We examined the structure of a 28-mer peptide (M1Lnk) representing a putative linker region between the N- and C-terminal domains of M1 by using circular dichroism, Raman, and absorption spectroscopy. M1Lnk assumes an α-helical structure in a mildly hydrophobic environment irrespective of pH, being consistent with the X-ray crystal structures of an N-terminal fragment of M1 at pH 7 and 4. In the presence of Zn2+, on the other hand, M1Lnk takes a partially unfolded conformation at neutral pH with a tetrahedral coordination of two Cys residues and two His residues to a Zn2+ ion in the central part of the peptide. Upon acidification, the peptide releases the Zn2+ ion and refolds into the α-helix-rich structure with a midpoint of transition at pH 5.9. The pH-dependent conformational transition of M1Lnk strongly suggests that the interdomain linker region of MI also undergoes a pH-dependent unfolding-refolding transition in the presence of Zn2+. A small but significant portion of the M1 protein is bound to Zn2+ in the virion, and the Zn2+-bound M1 molecule may play a special role in virus uncoating by changing the disposition of the N- and C-terminal domains upon acidification of the virion interior.
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