Membrane topology of Alzheimer's disease-related presenilin 1 - Evidence for the existence of a molecular species with a seven membrane-spanning and one membrane-embedded structure

A significant member of early-onset familial type of Alzheimer's disease ca ses has been shown to be caused by dominant mutations in either of the two genes encoding presenilin 1 (PS1) and presenilin 2 (PS2). These two protein s are highly homologous to each other and have been reported to be mainly l ocalized to the membranes of intracellular compartments such as the endopla smic reticulum. Information about the membrane topological structures of th ese proteins is indispensable for understanding their physiological and pat hological roles. Although several models have been proposed previously, the ir precise membrane topologies remain unknown. In this study, we examined t his issue in detail by expressing a series of C-terminally deleted PS1 muta nts fused to the hydrophilic portion of Escherichia coli leader peptidase i n vitro using a reticulocyte lysate in the presence of microsomal membranes . Our results predict that PS1 exists mainly in a seven membrane-spanning s tructure with its C-terminal end exposed to the luminal space. This was als o confirmed by expressing these fusion proteins in cultured cells, We furth er showed that a ninth hydrophobic segment is tightly bound to the membrane without spanning it. Based on the above observations, we propose a novel " seven membrane-spanning and one membrane-embedded" topological model for pr esenilins.