Calcium affinity, cooperativity, and domain interactions of extracellular EF-hands present in BM-40.
The structure and function of cytosolic Ca(2+)-binding proteins containing EF-hands are well understood. Recently, the presence of EF-hands in an extracellular protein was for the first time proven by the structure determination of the EC domain of BM-40 (SPARC (for secreted protein acidic and rich in cysteine)/osteonectin) (Hohenester, E., Maurer, P., Hohenadl, C., Timpl, R., Jansonius, J. N., and Engel, J. (1996) Nat. Struct. Biol. 3, 67-73). The structure revealed a pair of EF-hands with two bound Ca(2+) ions. Two unusual features were noted that distinguish the extracellular EF-hands of BM-40 from their cytosolic counterparts. An insertion of one amino acid into the loop of the first EF-hand causes a variant Ca(2+) coordination, and a disulfide bond connects the helices of the second EF-hand. Here we show that the extracellular EF-hands in the BM-40 EC domain bind Ca(2+) cooperatively and with high affinity. The EC domain is thus in the Ca(2+)-saturated form in the extracellular matrix, and the EF-hands play a structural rather than a regulatory role. Deletion mutants demonstrate a strong interaction between the EC domain and the neighboring FS domain, which contributes about 10 kJ/mol to the free energy of binding and influences cooperativity. This interaction is mainly between the FS domain and the variant EF-hand 1. Certain mutations of Ca(2+)-coordinating residues changed affinity and cooperativity, but others inhibited folding and secretion of the EC domain in a mammalian cell line. This points to a function of EF-hands in extracellular proteins during biosynthesis and processing in the endoplasmic reticulum or Golgi apparatus.
Busch
E
E
Institute for Biochemistry, Medical Faculty, University of Cologne, Germany.
Hohenester
E
E
Timpl
R
R
Paulsson
M
M
Maurer
P
P
D020816:Amino Acid Motifs; D002118:Calcium / Q000378:metabolism* / Q000502:physiology; D002460:Cell Line; D002942:Circular Dichroism; D016895:Culture Media, Serum-Free / Q000378:metabolism; D004591:Electrophoresis, Polyacrylamide Gel; D004721:Endoplasmic Reticulum / Q000378:metabolism; D006056:Golgi Apparatus / Q000378:metabolism; D006801:Humans; D015151:Immunoblotting; D007700:Kinetics; D008958:Models, Molecular; D016296:Mutagenesis; D015676:Osteonectin / Q000737:chemistry / Q000378:metabolism*; D017354:Point Mutation; D011485:Protein Binding; D011487:Protein Conformation; D017510:Protein Folding; D020836:Protein Structure, Quaternary; D017434:Protein Structure, Tertiary; D012333:RNA, Messenger / Q000378:metabolism; D020133:Reverse Transcriptase Polymerase Chain Reaction; D013050:Spectrometry, Fluorescence; D014162:Transfection
D016895:Culture Media, Serum-Free; D015676:Osteonectin; D012333:RNA, Messenger; D002118:Calcium