A Methionine-Rich Repeat Forms a Spiral Conformation That Guides Aragonite Nanofiber Organization in Molluscan Ligaments

The hinge ligament of Pinctadafucata contains aragonite nanofibers embedded in a dense organic matrix primarily composed of ligament methionine (Met)-rich protein (LMP). LMP features a low-complexity region with 30 repeats of the Met-Met-Met-lysine-proline-aspartic acid (MMMKPD) sequence; however, its structural and functional roles remain unclear. Using synthetic peptides and solution nuclear magnetic resonance with dispersive aragonite particles, we observed that the MMMKPD repeat formed a unique spiral conformation distinct from canonical secondary structures, which was supported by AlphaFold predictions. This structure is stabilized by Met-driven hydrophobic interactions and facilitates the electrostatic alignment of the charged side chains for mineral binding. The spiral structure may represent a previously unrecognized structural motif adapted to mechanically dynamic or high-pressure environments, such as hinge ligaments. Our findings suggest a structural mechanism through which LMP directs aragonite nanofiber formation in molluscan biominerals.