BACE1 is a membrane-bound aspartyl protease involved in production of the Alzheimer's amyloid beta-protein. The BACE1 ectodomain is partially cleaved to generate soluble BACE1, but the physiological significance of this event is unclear. During our characterization of BACE1 shedding from human neuroblastorna SH-SY5Y cells stably expressing BACE1, we unexpectedly found that detectable amounts of BACE1 holoproteins were released extracellularly along with soluble BACE1. Treatment with the metalloprotease inhibitor, TAPI-1, inhibited BACE1 shedding but increased BACE1 holoprotein release. Soluble and full-length BACE1 were released in parallel, at least partly originating from the plasma membrane. Furthermore, the release of soluble BACE1, but not full-length BACE1, was increased by deletion of the C-terminal dileucine motif, indicating that dysregulated BACE1 sorting affects BACE1 shedding. These findings suggest that the release of BACE1 holoproteins may be a physiologically relevant cellular process. (c) 2005 Elsevier Inc. All rights reserved.