Ubiquitously expressed secretory carrier membrane proteins (SCAMPs) 1-4 mark different pathways and exhibit limited constitutive trafficking to and from the cell surface

Abstract

Secretory carrier membrane proteins (SCAMPs) 1-4 are ubiquitously expressed and are major components of the eukaryotic cell surface recycling system. We investigated whether different SCAMPs function along distinct pathways and whether they behave like itinerant cargoes or less mobile trafficking machinery. In NRK cells, we show by immunofluorescence microscopy that different SCAMPs are concentrated mostly adjacent to one another in the trans-Golgi network and endosomal recycling compartment. By immunoelectron microscopy, they were shown to be close neighbors on individual transferrin-containing endosomal elements and on the plasma membrane. Within the internal endosomal network, SCAMPs are located distal to rab5-containing endosomes, and the individual isoforms appear to mark pathways that diverge from the constitutive recycling route and that may be distinguished by different adaptors, especially AP-1 and AP-3. Based on comparisons of SCAMP localization with endocytosed transferrin as well as live imaging of GFP-SCAMP1, we show that SCAMPs are concentrated within the motile population of early and recycling endosomes; however, they are not detected in newly formed transferrin-containing endocytic vesicles or in vesicles recycling transferrin to the surface. Also, they are not detected in constitutive secretory carriers marked by VSV-G. Their minimal recycling to the surface is reflected by their inability to relocate to the plasma membrane upon inhibition of endocytosis. Thus SCAMPs exhibit limited exchange between the cell surface and internal recycling systems, but within each of these sites, they form a mosaic with individual isoforms marking distinct pathways and potentially functioning as trafficking machinery at sites of vesicle formation and fusion. A corollary of these findings is that early endosomes exist as a distinct SCAMP-containing compartment and are not formed de novo by fusion of endocytic vesicles.