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Processing of Ameloblastin by MMP-20

¹ Department of Biologic and Materials Sciences and
² Department of Orthodontics and Pediatric Dentistry, Dental Research Lab, University of Michigan School of Dentistry, 1210 Eisenhower Place, Ann Arbor, MI 48108, USA;
³ Department of Hard Tissue Engineering, Tokyo Medical and Dental University, Tokyo, Japan; and
⁴ Department of Cytokine Biology, The Forsyth Institute Boston, MA, USA

^* corresponding author, jsimmer{at}umich.edu

Ameloblastin (AMBN) cleavage products are the most abundant non-amelogenin proteins in the enamel matrix of developing teeth. AMBN N-terminal cleavage products accumulate in the sheath space between enamel rods, while AMBN C-terminal products localize within rods. We tested the hypothesis that MMP-20 is the protease that cleaves AMBN. Glycosylated recombinant porcine AMBN (rpAMBN) was expressed in human kidney 293F cells, and recombinant porcine enamelysin (rpMMP-20) was expressed in bacteria. The purified proteins were incubated together at an enzyme:substrate ratio of 1:100. N-terminal sequencing of AMBN digestion products determined that rpMMP-20 cleaved rpAMBN after Pro², Gln¹³⁰, Gln¹³⁹, Arg¹⁷⁰, and Ala²²². This shows that MMP-20 generates the 23-kDa AMBN starting at Tyr²²³, as well as the 17-kDa (Val¹-Arg¹⁷⁰) and 15-kDa (Val¹-Gln¹³⁰) AMBN cleavage products that concentrate in the sheath space during the secretory stage. We conclude that MMP-20 processes ameloblastin in vitro and in vivo.

KEY WORDS: enamelysin • ameloblastin • AMBN • MMP-20 • enamel • amelogenesis

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