Building with melanin: Low complexity Glycera jaw protein is master of multitasking

In protein structure-function relationships, there is a widely held view that complex functions require complex structures. Recent studies with a low complexity protein from the jaws of Glycera, a benthic marine polychaete, appear to challenge this. Used for grasping and injecting venom, the jaws are stiff, sharp, durable and wear resistant. Proteins (50 wt %) are one of three jaw components including melanin (40 wt %) and copper (10 wt %) present as a copper mineral (atacamite) and Cu²⁺ ions. Although jaw stiffness is correlated with melanin not mineral, little was known about the protein contribution until its recent characterization and recombinant expression. The Glycera jaw protein has an extremely low complexity sequence in which GGH repeats represent 80% of the composition. Notwithstanding such monotony, GJP exhibits the following distinct properties: 1) it binds 26 equivalents of Cu per protein, 2) catalyzes oxidation of Dopa to Dopa-quinone en route to melanin, 3) templates a sheet-like assembly (32 nm thick) of melanin at the air-water interface, and 4) forms a structural network with melanin and Cu in the stacked sheets of higher order architecture.