Thermal Stability of Individual Domains in Platelet Glycoprotein IIbIIIa

Thermal denaturation of platelet glycoprotein IIbIIIa (integrin alpha IIb beta 3) was investigated by spectrofluorimetry and differential scanning calorimetry (DSC). Two forms of the protein were compared: active IIbIIIa, i.e., that fraction that binds to RGD-Sepharose, and inactive IIbIIIa, the non-binding fraction. At pH 8.5 in the presence of octyl glucoside and Ca2+ both forms exhibited a broad complex endotherm consisting of a well expressed low-temperature heat-absorption peak in the range of 40-65 degrees C followed by a broad peak stretching over 65-110 degrees C. Each endotherm could be deconvoluted into at least eight transitions reflecting the melting of at least this many independently folded domains. The first two transitions in the region of the low-temperature peak had similar positions in both forms while at least some of the other transitions occurred at higher temperature in the active protein suggesting that some of the domains are more stable in the latter. When both fractions of IIbIIIa were heated in the fluorometer a sigmoidal transition was observed in the region of the first endothermic peak where the two thermolabile domains melt. This transition was destabilized by 15 degrees C in the presence of EDTA, suggesting that these domains are formed by the 243-468 region of the IIb subunit which contains four Ca(2+)-binding motifs. It was further stabilized by 3 degrees C upon addition of the GRGDSPK peptide in the presence of Ca2+ while in EDTA the peptide had no effect. This is consistent with the involvement of Ca(2+)-binding region in the formation of the ligand-binding site. A 66-kDa chymotryptic fragment, containing the 17-kDa NH2-terminal portion of the IIIa subunit disulfide-linked to its 50-kDa COOH-terminal portion including the cysteine-rich core, exhibited a fluorescence-detected Ca(2+)-independent transition in the region where the higher temperature DSC-detected transitions occur suggesting that some of the latter may be connected with the melting of the corresponding portions of IIbIIIa.