Calcium binding studies of peptides of human phospholipid scramblases 1 to 4 suggest that scramblases are new class of calcium binding proteins in the cell.
BACKGROUND
Phospholipid scramblases are a group of four homologous proteins conserved from C. elegans to human. In human, two members of the scramblase family, hPLSCR1 and hPLSCR3 are known to bring about Ca2+ dependent translocation of phosphatidylserine and cardiolipin respectively during apoptotic processes. However, affinities of Ca2+/Mg2+ binding to human scramblases and conformational changes taking place in them remains unknown.


METHODS
In the present study, we analyzed the Ca2+ and Mg2+ binding to the calcium binding motifs of hPLSCR1-4 and hPLSCR1 by spectroscopic methods and isothermal titration calorimetry.


RESULTS
The results in this study show that (i) affinities of the peptides are in the order hPLSCR1>hPLSCR3>hPLSCR2>hPLSCR4 for Ca2+ and in the order hPLSCR1>hPLSCR2>hPLSCR3>hPLSCR4 for Mg2+, (ii) binding of ions brings about conformational change in the secondary structure of the peptides. The affinity of Ca2+ and Mg2+ binding to protein hPLSCR1 was similar to that of the peptide I. A sequence comparison shows the existence of scramblase-like motifs among other protein families.


CONCLUSIONS
Based on the above results, we hypothesize that the Ca2+ binding motif of hPLSCR1 is a novel type of Ca2+ binding motif.


CONCLUSIONS
Our findings will be relevant in understanding the calcium dependent scrambling activity of hPLSCRs and their biological function.
Sahu
Santosh Kumar
SK
Department of Biotechnology, Indian Institute of Technology--Madras, Chennai 600 036, India.
Aradhyam
Gopala Krishna
GK
Gummadi
Sathyanarayana N
SN
D000595:Amino Acid Sequence; D001665:Binding Sites; D001667:Binding, Competitive; D002118:Calcium / Q000378:metabolism*; D002135:Calcium-Binding Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D002942:Circular Dichroism; D031541:Fluorescence Resonance Energy Transfer; D006801:Humans; D008274:Magnesium / Q000378:metabolism; D008969:Molecular Sequence Data; D010455:Peptides / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D049294:Phospholipid Transfer Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D011485:Protein Binding; D011487:Protein Conformation; D017386:Sequence Homology, Amino Acid; D013725:Terbium / Q000737:chemistry / Q000378:metabolism; D014364:Tryptophan / Q000737:chemistry / Q000378:metabolism
D002135:Calcium-Binding Proteins; C493422:PLSCR1 protein, human; C493423:PLSCR2 protein, human; C493424:PLSCR3 protein, human; C493425:PLSCR4 protein, human; D010455:Peptides; D049294:Phospholipid Transfer Proteins; D013725:Terbium; D014364:Tryptophan; D008274:Magnesium; D002118:Calcium