The polytopic membrane protein presenilin 1 (PS1) is a component of the gamma-secretase complex that is responsible for the intramembranous cleavage of several type I transmembrane proteins, including the beta-amyloid precursor protein (APP). Mutations of PS1, apparently leading to aberrant processing of APP, have been genetically linked to early-onset familial Alzheimer's disease. PS1 contains 10 hydrophobic regions (HRs) sufficiently long to be alpha-helical membrane spanning segments. Most topology models for PS1 place its COOH terminal approximately 40 amino acids, which include HR 10, in the cytosolic space. However, several recent observations suggest that HR 10 may be integrated into the membrane and involved in the interaction between PS1 and APP. We have applied three independent methodologies to investigate the location of HR 10 and the extreme COOH terminus of PS1. The results from these methods indicate that HR 10 spans the membrane and that the COOH terminal amino acids of PS1 lie in the extracytoplasmic space.