Release of nontransmembrane full-length Alzheimer's amyloid precursor protein from the lumenar surface of chromaffin granule membranes.

Abstract

We previously demonstrated the presence of a soluble form of full-length Alzheimer's amyloid precursor protein (APP) in the lumen of adrenal medullary chromaffin granules (CG). Furthermore, full-length APP is released from CG membranes in vitro at pH 9.0 by an enzymatic mechanism, sensitive to protease inhibitors [Vassilacopoulou et al. (1995) J. Neurochem. 64, 2140-2146]. In this study, we found that when intact CG were subjected to exogenous trypsin, a fraction of APP was not digested, consistent with an intragranular population of APP. To examine the substrate-product relationship between membrane and soluble full-length APP, we labeled CG transmembrane APP with 3-(trifluoromethyl)-3-(m-[125I]iodophenyl)diazirine ([125I]TID), a lipophilic probe, specific for membrane-spanning domains of proteins. APP released from the membranes at pH 9.0 was not labeled with [125I]TID. In addition, this APP was not biotinylated in intact CG. Combined, the results indicate that APP released from CG membranes derives from a unique nontransmembrane population of membrane-associated APP, located in the lumenal side of CG membranes. Dithiobis(succinimidylpropionate) (DSP) cross-linking indicated that APP in CG is situated in close proximity with other proteins, possibly with APP itself. APP complexes were also detected under nonreducing conditions, without DSP cross-linking. These results, combined with our previous studies, indicate that full-length APP within CG exists as three different populations: (I) transmembrane, (II) membrane-associated/nontransmembrane, and (III) soluble. The existence of nontransmembrane populations suggests that putative gamma-secretase cleavage sites of APP, assumed to be buried within the lipid bilayer, could be accessible to proteolysis in a soluble intravesicular environment.

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@article{Tezapsidis1998ReleaseON, title={Release of nontransmembrane full-length Alzheimer's amyloid precursor protein from the lumenar surface of chromaffin granule membranes.}, author={Nikolaos Tezapsidis and Haizhong Li and James A. Ripellino and Spiros Efthimiopoulos and Dido Vassilacopoulou and Kumar Sambamurti and Thomas Toneff and Sukkid Yasothornsrikul and Vivian Hook and Nikolaos K. Robakis}, journal={Biochemistry}, year={1998}, volume={37 5}, pages={1274-82} }