The full-length human AM and its functional fragments, TRAP and LRAP, were reassembled and purified in vitro. The protein solution of 100 µg‧mL^-1, pH=8, was prepared in Tris-HCl and incubated at room temperature for 1-15 min. Their self-assembly behaviors were observed and compared under a transmission electron microscope (TEM). The full-length AM was added to artificial saliva and incubated for 3 days. A scanning electron microscope (SEM) was used in observing the morphology of the induced new crystals. Then, TARP and LRAP were added. The resulting solution was incubated for 3 days and then observed again.

When pH=8, the full-length human AM and TRAP assembly started spontaneously and formed “nanospheres” after 15 min.The nanospheres formed by TRAP existed independently, with a uniform size but without obvious internal structures. The full-length AM was assembled hierarchically, which formed “nanospheres” and further extended in all directions, formed a chain structure, and then aggregated into a net. The self-assembly behavior of LRAP was not obvious. Proteins mostly existed in the form of monomers without “nanosphere” formation. Only few oligomers were observed. The full-length AM was induced independently for 3 days to form rod-shaped HA crystals. TRAP and LRAP proteins were added, after 3 days the crystal elongation was obvious in the c axis, but the growth in plane A and plane B was poor.

The self-assembly and mineralization behaviors of full-length human AM, TRAP, and LRAP were consistent with the directional growth mechanism of HA crystals in vivo, providing a theoretical basis for the role of the fragments in the growth and maturation of HA crystals.