Slcllal is an integral membrane protein with 12 predicted transmembrane domams（TM）. In the presentwork, the secondary structures and positioning of the peptides associated with SIclIaI-TM2, TM3 andTM4（wild-type peptides and fimctional mutants） in sodium dodecyl sulfate（SDS） were analyzed using circulardichroism（CD） and fluorescence techniques. The results indicate that TM3 is significantly different in secondarystructure and topology from TM2 and TM4 in SDS. The peptide TM3 is less structured and is embedded in the SDSless deeply than TM2 and TM4 at pH=4, 5.5 and 7. In addition, the position of TM3 is remarkably shifted towards thedirection of the SDS surface with increasing pH, whereas the locations of TM2 and TM4 in SDS are less affected bypH. The E 139A substitution in TM3 significantly impairs the pH dependence of the buried depth of TM3 and causes adecrease in helicity in SDS at the three pH values. The G169D mutation has little effect on the topological arrange-ment of TM4 in SDS. In contrast, TM2 and TM4 are topologically similar.
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