ALTERATIONS IN MEMBRANE-PROTEIN CONFORMATION IN RESPONSE TO SENESCENCE-RELATED CHANGES

Measurements of diphenylhexatriene fluorescence polarization in smooth microsomal membranes isolated from senescing bean cotyledons have indicated that membrane bulk lipid fluidity decreases by ca two-fold as the cotyledons age. This change in fluidity can be largely attributed to an increase in relative sterol concentration (from ca 7 to ca 45 mol% with respect to phospholipid) and is accompanied by a change in the average conformation of membrane proteins detectable by spin labelling with 3-maleimido-PROXYL. When the sterol concentration of smooth microsomal membranes from young cotyledons was increased in vitro by treatment with cholesteryl hemisuccinate, the membrane lipid fluidity was reduced to a value comparable to that for corresponding membranes from senescent cotyledons. The cholesteryl hemisuccinate treatment also induced conformational changes in the membrane proteins, but did not alter protein composition as shown by SDS-PAGE. The observations collectively indicate that the sterol-induced decrease in bulk lipid fluidity of smooth microsomal membranes accompanying senescence is of sufficient magnitude to alter membrane protein conformation. These changes in protein conformation may also reflect sterol-induced protein aggregation, which is known to occur at relative sterol concentrations in excess of ca 27 mol% with respect to phospholipid.