Mannose 6-phosphate independent membrane association and targeting of procathepsin D

Date of Award




Degree Name

Doctor of Philosophy (Ph.D.)


Cell Biology and Anatomy

First Committee Member

Gregory E. Conner - Committee Chair

Second Committee Member

Nevis Fregien - Committee Member


Lysosomal enzymes are targeted to the lysosome mainly by the Mannose-6-phosphate (m6p) dependent pathway. However, m6p independent pathways also exist for some lysosomal enzymes and in certain cell types, although the mechanism remains unclear. Recent observation that several soluble lysosomal enzyme precursors can transiently associate with membranes independent of m6p raises the possibility that this phenomenon may be related to the m6p independent targeting pathway. To investigate the mechanism of the m6p independent membrane association and targeting of procathepsin D, membrane associated procathepsin D was cross-linked with a reversible chemical cross-linker to explore possible membrane components (proteins, or receptors) that may associate with it. It was found that procathepsin D can specifically interact with another lysosomal protein precursor, prosaposin, to form a protein complex immediately after synthesis in ER. The proteins in the complex, however, did not bind to the ER membrane. Membrane binding began in the late Golgi, in both a m6p dependent and an independent manner. Upon reaching the late endosomes or heavy lysosomes where proteolytic processing of lysosomal proenzymes is supposed to occur, procathepsin D and prosaposin began to dissociate from each other and from the membrane. Prosaposin could bind to the membranes independent of m6p and seemed to be required for the membrane association of procathepsin D in the procathepsin D-prosaposin complex. In cells deficient in prosaposin, procathepsin D could no longer associate with the membranes. Cell fractionation studies of I-cell fibroblasts suggested that only the procathepsin D-prosaposin complex can reach heavy subcellular compartments independent of m6p. Membrane association and biosynthetic studies of procathepsin D in normal and prosaposin deficient fibroblasts also indicated that the procathepsin D-prosaposin complex facilitated the m6p dependent targeting of both proteins.


Biology, Molecular; Biology, Cell

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