Crystal structure of a secreted insect ferritin reveals a symmetrical arrangement of heavy and light chains.
Ferritins are iron storage proteins made of 24 subunits forming a hollow spherical shell. Vertebrate ferritins contain varying ratios of heavy (H) and light (L) chains; however, known ferritin structures include only one type of chain and have octahedral symmetry. Here, we report the 1.9A structure of a secreted insect ferritin from Trichoplusia ni, which reveals equal numbers of H and L chains arranged with tetrahedral symmetry. The H/L-chain interface includes complementary features responsible for ordered assembly of the subunits. The H chain contains a ferroxidase active site resembling that of vertebrate H chains with an endogenous, bound iron atom. The L chain lacks the residues that form a putative iron core nucleation site in vertebrate L chains. Instead, a possible nucleation site is observed at the L chain 3-fold pore. The structure also reveals inter- and intrasubunit disulfide bonds, mostly in the extended N-terminal regions unique to insect ferritins. The symmetrical arrangement of H and L chains and the disulfide crosslinks reflect adaptations of insect ferritin to its role as a secreted protein.
Hamburger
Agnes E
AE
Division of Biology 114-96, California Institute of Technology, Pasadena, CA 91125, USA.
West
Anthony P
AP
Hamburger
Zsuzsa A
ZA
Hamburger
Peter
P
Bjorkman
Pamela J
PJ
D000595:Amino Acid Sequence; D000818:Animals; D003198:Computer Simulation; D003553:Cystine / Q000737:chemistry / Q000235:genetics; D005290:Ferric Compounds; D005293:Ferritins / Q000737:chemistry* / Q000235:genetics / Q000302:isolation & purification / Q000378:metabolism; D006801:Humans; D008958:Models, Molecular; D008969:Molecular Sequence Data; D009036:Moths / Q000378:metabolism; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary
D005290:Ferric Compounds; D003553:Cystine; C092844:ferric oxyhydroxide; D005293:Ferritins