Glassin, a histidine-rich protein from the siliceous skeletal system of the marine sponge Euplectella, directs silica polycondensation.
The hexactinellids are a diverse group of predominantly deep sea sponges that synthesize elaborate fibrous skeletal systems of amorphous hydrated silica. As a representative example, members of the genus Euplectella have proved to be useful model systems for investigating structure-function relationships in these hierarchically ordered siliceous network-like composites. Despite recent advances in understanding the mechanistic origins of damage tolerance in these complex skeletal systems, the details of their synthesis have remained largely unexplored. Here, we describe a previously unidentified protein, named "glassin," the main constituent in the water-soluble fraction of the demineralized skeletal elements of Euplectella. When combined with silicic acid solutions, glassin rapidly accelerates silica polycondensation over a pH range of 6-8. Glassin is characterized by high histidine content, and cDNA sequence analysis reveals that glassin shares no significant similarity with any other known proteins. The deduced amino acid sequence reveals that glassin consists of two similar histidine-rich domains and a connecting domain. Each of the histidine-rich domains is composed of three segments: an amino-terminal histidine and aspartic acid-rich sequence, a proline-rich sequence in the middle, and a histidine and threonine-rich sequence at the carboxyl terminus. Histidine always forms HX or HHX repeats, in which most of X positions are occupied by glycine, aspartic acid, or threonine. Recombinant glassin reproduces the silica precipitation activity observed in the native proteins. The highly modular composition of glassin, composed of imidazole, acidic, and hydroxyl residues, favors silica polycondensation and provides insights into the molecular mechanisms of skeletal formation in hexactinellid sponges.
Division of Regional Contribution and Lifelong Learning, Organization for Regional Industrial Academic Cooperation, Tottori University, Tottori 680-8550, Japan; kshimizu@cjrd.tottori-u.ac.jp.
Katsuhiko
K
Shimizu
Department of Agricultural, Biological and Environmental Science, Faculty of Agriculture, Tottori University, Tottori 680-8553, Japan;
Taro
T
Amano
Department of Agricultural, Biological and Environmental Science, Faculty of Agriculture, Tottori University, Tottori 680-8553, Japan;
Md Rezaul
MR
Bari
Wyss Institute for Biologically Inspired Engineering, Harvard University, Cambridge, MA 01238.
James C
JC
Weaver
Department of Agricultural, Biological and Environmental Science, Faculty of Agriculture, Tottori University, Tottori 680-8553, Japan;
Jiro
J
Arima
Department of Agricultural, Biological and Environmental Science, Faculty of Agriculture, Tottori University, Tottori 680-8553, Japan;
Nobuhiro
N
Mori
D000595:Amino Acid Sequence; D000596:Amino Acids / Q000737:chemistry; D000818:Animals; D001224:Aspartic Acid / Q000737:chemistry; D001665:Binding Sites; D003001:Cloning, Molecular; D018076:DNA, Complementary / Q000737:chemistry; D004591:Electrophoresis, Polyacrylamide Gel; D000939:Epitopes / Q000737:chemistry; D005843:Geography; D006639:Histidine / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D006868:Hydrolysis; D008969:Molecular Sequence Data; D010455:Peptides / Q000737:chemistry; D011161:Porifera / Q000737:chemistry*; D011392:Proline / Q000737:chemistry; D011499:Protein Processing, Post-Translational; D011506:Proteins / Q000737:chemistry* / Q000235:genetics; D011994:Recombinant Proteins / Q000737:chemistry; D017386:Sequence Homology, Amino Acid; D012822:Silicon Dioxide / Q000737:chemistry*; D012995:Solubility; D013696:Temperature; D013912:Threonine / Q000737:chemistry; D000099053:Histidine-Rich Glycoprotein
D000596:Amino Acids; D018076:DNA, Complementary; D000939:Epitopes; D010455:Peptides; D011506:Proteins; D011994:Recombinant Proteins; D000099053:Histidine-Rich Glycoprotein; D013912:Threonine; D001224:Aspartic Acid; D006639:Histidine; D012822:Silicon Dioxide; D011392:Proline
Porifera; biomineral; fusion materials; organic–inorganic composite; silicon dioxide