The toposome, essential for sea urchin cell adhesion and development, is a modified iron-less calcium-binding transferrin.
We describe the structure and function of the toposome, a modified calcium-binding, iron-less transferrin, the first member of a new class of cell adhesion proteins. In addition to the amino acid sequence of the precursor, we determined by Edman degradation the N-terminal amino acid sequences of the mature hexameric glycoprotein present in the egg as well as that of its derived proteolytically modified fragments necessary for development beyond the blastula stage. The approximate C-termini of the fragments were determined by a combination of mass spectrometry and migration in reducing gels before and after deglycosylation. This new member of the transferrin family shows special features which explain its evolutionary adaptation to development and adhesive function in sea urchin embryos: (i) a protease-inhibiting WAP domain, (ii) a 280 amino acid cysteine-less insertion in the C-terminal lobe, and (iii) a 240 residue C-terminal extension with a modified cystine knot motif found in multisubunit external cell surface glycoproteins. Proteolytic removal of the N-terminal WAP domain generates the mature toposome present in the oocyte. The modified cystine knot motif stabilizes cell-bound trimers upon Ca-dependent dissociation of hexamer-linked cells. We determined the positions of the developmentally regulated cuts in the cysteine-less insertion, which produce the fragments observed previously. These fragments remain bound to the hexameric 22S particle in vivo and are released only after treatment of the purified toposome with reducing agents. In addition, some soluble smaller fragments with possible signal function are produced. Sequence comparison of five sea urchin species reveals the location of the cell-cell contact site targeted by the species-specific embryo dissociating antibodies. The evolutionary tree of 2-, 1-, and 0-ferric transferrins implies their evolution from a basic cation-activated allosteric design modified to serve multiple functions.
Noll
Hans
H
Department of Cell and Molecular Biology, University of Hawaii, School of Medicine, 1960 East-West Road, Honolulu, HI 96822, USA. hansnoll1@comcast.net
Alcedo
Joy
J
Daube
Michael
M
Frei
Erich
E
Schiltz
Emile
E
Hunt
John
J
Humphries
Tom
T
Matranga
Valeria
V
Hochstrasser
Martin
M
Aebersold
Ruedi
R
Lee
Hookeun
H
Noll
Markus
M
D000595:Amino Acid Sequence; D000818:Animals; D002135:Calcium-Binding Proteins / Q000235:genetics* / Q000378:metabolism; D002448:Cell Adhesion; D004625:Embryo, Nonmammalian / Q000166:cytology / Q000378:metabolism; D006023:Glycoproteins / Q000235:genetics* / Q000378:metabolism; D007501:Iron; D008024:Ligands; D008969:Molecular Sequence Data; D010802:Phylogeny; D011498:Protein Precursors / Q000235:genetics / Q000378:metabolism; D017434:Protein Structure, Tertiary / Q000235:genetics; D012617:Sea Urchins / Q000196:embryology / Q000235:genetics* / Q000378:metabolism; D016415:Sequence Alignment; D017421:Sequence Analysis; D013329:Structure-Activity Relationship; D014168:Transferrin / Q000235:genetics* / Q000378:metabolism
D002135:Calcium-Binding Proteins; D006023:Glycoproteins; D008024:Ligands; D011498:Protein Precursors; D014168:Transferrin; C118907:toposome glycoprotein complex; D007501:Iron