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.