The lumenal domain of the integral membrane protein phogrin mediates targeting to secretory granules.
Phogrin, a transmembrane glycoprotein of neuroendocrine cells, is localized to dense-core secretory granules. We have investigated the subcellular targeting of phogrin by analyzing the sorting of a series of deletion mutants to the regulated pathway of secretion in AtT20 cells. The lumenal domain as a soluble protein was efficiently routed to granules, based on a combination of morphological analysis and secretion studies. Sorting was not dependent on a candidate targeting signal consisting of an N-terminal conserved cysteine-rich motif. Both the pro-region and the lumenal domain of mature, post-translationally processed phogrin independently reached the granule, although the pro-region was sorted more efficiently. Once within the regulated secretory pathway, all phogrin lumenal domain proteins were stored in functional granules for extended periods of time. Thus, phogrin possesses several domains contributing to its targeting to the secretory granule. Our findings support a model of granule biogenesis where proteins are sorted on the basis of their biochemical properties rather than via signal-dependent binding to a targeting receptor. Sorting of integral membrane proteins mediated by the lumenal domain may ensure that functionally important transmembrane molecules are included in the forming granule.
Barbara Davis Center for Childhood Diabetes, University of Colorado Health Sciences Center, 4200 East 9th Avenue, Box B140, Denver, CO 80262, USA.
Christina
C
Wasmeier
Nicholas A
NA
Bright
John C
JC
Hutton
D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D002460:Cell Line; D003513:Cycloheximide / Q000494:pharmacology; D017931:DNA Primers; D006801:Humans; D008565:Membrane Proteins / Q000737:chemistry / Q000378:metabolism*; D051379:Mice; D008854:Microscopy, Electron; D008856:Microscopy, Fluorescence; D008969:Molecular Sequence Data; D017027:Protein Tyrosine Phosphatases / Q000737:chemistry / Q000378:metabolism*; D054635:Receptor-Like Protein Tyrosine Phosphatases, Class 8; D011994:Recombinant Proteins / Q000737:chemistry / Q000378:metabolism; D017386:Sequence Homology, Amino Acid
D017931:DNA Primers; D008565:Membrane Proteins; D011994:Recombinant Proteins; D003513:Cycloheximide; C102889:PTPRN2 protein, human; D017027:Protein Tyrosine Phosphatases; D054635:Receptor-Like Protein Tyrosine Phosphatases, Class 8