Cysteine String Proteins Are Associated with Chromaffin Granules

In this work, we have examined the subcellular distribution of cysteine string proteins (Csps) in bovine adrenal medullary chromaffin cells. Csps did not leak from digitonin-permeabilized chromaffin cells, suggesting that there is no cytosolic pool of the protein in these cells. Subcellular fractionation studies confirmed that there was essentially no Csp immunoreactivity in the cytosolic fraction. However, immunoreactivity was detected in the membrane fractions of these cells. Csp immunoreactivity codistributed with dopamine beta-hydroxylase, a granule marker protein, in sucrose gradient-separated granule fractions. Immunofluorescence studies showed that all chromaffin cells in culture were stained with a punctate appearance consistent with a granular localization. These results were confirmed by immunogold labeling, which demonstrated specific labeling of chromaffin granule membranes. In addition to its presence on synaptic vesicles, cysteine string protein is therefore a bona fide chromaffin granule membrane protein.