STRUCTURAL-PROPERTIES OF THE MICROSOMAL TRIGLYCERIDE-TRANSFER PROTEIN COMPLEX

The microsomal triglyceride-transfer protein (MTP), which catalyzes the transport of triglyceride and cholesteryl ester between membranes, is a complex composed of two proteins having apparent molecular weights of 58 000 and 88 000. The 58 000 molecular weight component of MTP has been identified as the multifunctional protein, protein disulfide isomerase (PDI). The multisubunit nature of MTP as well as the presence of PDI as one of the subunits distinguishes this protein from previously characterized lipid-transfer proteins. In this study, we have more clearly defined structural elements of MTP that may play important functional roles. The molecular weight of the transfer protein complex was determined to be 150 000 by sedimentation equilibrium experiments performed at three different speeds, suggesting that MTP is a complex of one PDI and one 88 000 molecular weight polypeptide (88K). Following SDS-polyacrylamide gel electrophoresis, the Coomassie Blue staining intensity of PDI in a known amount of MTP was compared to that of known amounts of a PDI standard. A 1 to 0.98-1.30 ratio of PDI to 88K was determined, confirming the 1:1 stoichiometry of MTP. The sedimentation coefficient (5.85) determined by analytical ultracentrifugation and the Stokes radius (47 angstrom) determined by polyacrylamide gradient gel electrophoresis indicate that the 150 000 molecular weight MTP complex is asymmetric and/or has an unusually high water of hydration. PDI and 88K form a stable protein complex; there was no evidence of a dissociation-reassociation reaction occurring between the two components. Analysis of far-ultraviolet circular dichroism spectra revealed MTP has about 28% alpha-helical and 28% beta-structural content. PDI appeared to be relatively rich in alpha-helical and random structure (31 and 34%, respectively), while 88K appeared rich in beta-structure (35%). The stability of MTP and its components was investigated by guanidine hydrochloride denaturation as monitored by circular dichroism spectroscopy. MTP was resistant to denaturation up to 0.8 M guanidine hydrochloride. PDI structure appeared to be stabilized by interaction with 88K. The 88-kDa subunit appeared stable at 1.2 M guanidine hydrochloride, when it is dissociated from PDI. Both components of MTP had broad denaturation transitions, indicative of stable folding intermediates. Collectively, these data imply that PDI is an important integral part of the lipid-transfer protein.