The thermal stability of cholera toxin free in solution and in association with its cell-surface receptor ganglioside GM1 has been studied by using high-sensitivity differential scanning calorimetry and differential solubility thermal gel analysis. In the absence of ganglioside GM1, cholera toxin undergoes two distinct thermally induced transitions centered at 51 and 74 degrees C, respectively. The low-temperature transition has been assigned to the irreversible thermal denaturation of the active A subunit. The second transition has been assigned to the reversible unfolding of the B subunit pentamer. The isolated B subunit pentamer exhibits a single transition also centered at 74 degrees C, suggesting that the attachment of the A subunit does not contribute to the stability of the pentamer. In the intact toxin, the A subunit dissociates from the B subunit pentamer at a temperature that coincides with the onset of the B subunit thermal unfolding. In aqueous solution, the denatured A subunit precipitates after dissociation from the B subunit pentamer. This phenomenon can be detected calorimetrically by the appearance of an exothermic heat effect. In the presence of ganglioside GM1, the B subunit is greatly stabilized as indicated by an increase of 20 degrees C in the transition temperature. In addition, ganglioside GM1 greatly enhances the cooperative interactions between B subunits. In the absence of ganglioside, each monomer within the B pentamer unfolds in an independent fashion whereas the fully ganglioside-bound pentamer behaves as a single cooperative unit. On the contrary, the thermotropic behavior of the A subunit is only slightly affected by the presence of increasing concentrations of ganglioside GM1.(ABSTRACT TRUNCATED AT 250 WORDS) Study holds ProTherm entries: 3884, 3885 Extra Details: additive : Na2EDTA(1 mM),NaN3(3 mM) was added in the experiment exothermic heat effect; cooperative interactions;,micellar; membrane-bound gangliosides
Submitter: Connie Wang
Submission Date: April 24, 2018, 8:23 p.m.
|Number of data points||5|
|Proteins||Type II secretion system protein E ; Cholera enterotoxin subunit B|
|Assays/Quantities/Protocols||Experimental Assay: dHcal ; Experimental Assay: Tm ; Experimental Assay: dHvH|
|Libraries||Mutations for sequence TPQNITDLCAEYHNTQIHTLNDKIFSYTESLADKREMAIITFKNGATFQVEVPGSQHIDSQKKAIERMKDTLRIAYLTEAKVEKLCVWNNKTPHAIAAISMAN|