CHARACTERIZATION OF GLYCOPROTEIN-II FROM BOVINE ADRENAL-MEDULLARY CHROMAFFIN GRANULES - IDENTIFICATION OF COMPONENTS REPRESENTING THE SECRETORY VESICLE COUNTERPARTS OF THE LYSOSOMAL-ASSOCIATED MEMBRANE-GLYCOPROTEINS (LAMP-1 AND LAMP-2)

Glycoprotein II (GpII) is a heterogenous glycoprotein isolated from th e membranes of bovine chromaffin granules in the adrenal medulla. When viewed by two-dimensional electrophoresis this glycoprotein consists of two components, upper (GpIIa) and lower (GpIIb), with a molecular m ass of 80,000-100,000 daltons and a pI of 4.2-4.7. NH2-terminal sequen ce analysis of GpIIa and GpIIb revealed sequence similarity with lysos omal membrane glycoproteins (lamp-1 and lamp-2), which was supported b y sequence data of peptides from trypsin and cyanogen bromide digestio ns. An oligonucleotide probe was used to isolate a cDNA clone encoding the nucleotide sequence of GpIIa. The predicted amino acid sequence o f GpIIa shares a 72% identity with the human lamp-1 type protein, whic h belongs to a highly conserved group of lysosomal-associated membrane glycoproteins (lamp proteins), whose function is still unknown. The C OOH-terminal region of GpIIa was identical to the COOH-terminal region of lamp proteins. This COOH-terminal determinant has been demonstrate d to be essential for the intracellular targeting of lamp proteins to lysosomes. A synthetic peptide antisera to the COOH-terminal region of GpIIa was used to show that this region is present on purified chroma ffin granules and not proteolytically processed. The sequence analysis of GpIIa and immunological data confirm GpII as the secretory granule counterpart of lamp proteins and raise some questions regarding intra cellular targeting between lysosomes and secretory granules within the chromaffin cell.