The C-terminus and the Ca2+ low-affinity binding sites in bacteriorhodopsin.
Bacteriorhodopsin (bR) is found previously to have two high-affinity and four to six low-affinity Ca2+ binding sites. Our previous studies with site-directed mutation suggested that the two high-affinity sites are located within the protein and close to the retinal pocket. In order to investigate the location of the four to six low-affinity binding sites, we studied Ca2+ binding to deionized bR as well as its variant in which its C-terminus is removed by papain treatment. Potentiometric titration with Ca(2+)-selective electrodes were carried out and Scatchard plots were obtained from the titration data. We found that most of the low-affinity sites are eliminated upon removal of the C-terminus in bR. This suggests that the low-affinity sites in bR are on the surface. The involvement of the C-terminus in these sites is discussed.
Zhang
N Y
NY
Department of Chemistry and Biochemistry, University of California, Los Angeles 90024.
el-Sayed
M A
MA
D001436:Bacteriorhodopsins / Q000737:chemistry*; D001665:Binding Sites; D002118:Calcium / Q000378:metabolism; D019617:Halobacterium salinarum / Q000737:chemistry; D010446:Peptide Fragments / Q000737:chemistry; D011485:Protein Binding
D010446:Peptide Fragments; D001436:Bacteriorhodopsins; D002118:Calcium