Selective delivery of secretory cargo in Golgi-derived carriers of nonepithelial cells

In epithelial cells, soluble cargo proteins destined for basolateral or apical secretion are packaged into distinct trans-Golgi network-derived transport carriers. Similar carriers, termed basolateral- and apical-like, have been observed in nonepithelial cells using ectopically expressed membrane marker proteins. Whether these cells are capable of selectively packaging secretory proteins into distinct carriers is still an open question. Here, we have addressed this issue by analyzing the packaging and transport of secretory human chromogranin B fusion proteins using a green fluorescent protein-based high-resolution, dual-color imaging technique. We were able to show that these secretory markers were selectively packaged at the Golgi into tubular/vesicular-like transport carriers containing basolateral membrane markers, resulting in extensive cotransport. In contrast, deletion mutants of the human chromogranin B fusion proteins lacking an N-terminal loop structure were efficiently transported in both basolateral- and apical-like carriers, the latter displaying a spherical morphology. Similarly, in polarized epithelial cells, the human chromogranin B fusion protein was secreted basolaterally and the loop-deleted analogue into both the basolateral and apical medium. These findings suggest that nonepithelial cells, like their epithelial counterparts, possess a sorting machinery capable of selective packaging of secretory cargo into distinct types of carriers.