The mammalian retromer protein complex, which consists of three proteins--Vps26, Vps29, and Vps35--in association with members of the sorting nexin family of proteins, has been implicated in the trafficking of receptors and their ligands within the endosomal/lysosomal system of mammalian cells. A bioinformatic analysis of the mouse genome identified an additional transcribed paralog of the Vps26 retromer protein, which we termed Vps26B. No paralogs were identified for Vps29 and Vps35. Phylogenetic studies indicate that the two paralogs of Vps26 become evident after the evolution of the chordates. We propose that the chordate Vps26-like gene published previously be renamed Vps26A to differentiate it from Vps26B. As for Vps26A, biochemical characterization of Vps26B established that this novel 336 amino acid residue protein is a peripheral membrane protein. Vps26B co-precipitated with Vps35 from transfected cells and the direct interaction between these two proteins was confirmed by yeast 2-hybrid analysis, thereby establishing Vps26B as a subunit of the retromer complex. Within HeLa cells, Vps26B was found in the cytoplasm with low levels at the plasma membrane, while Vps26A was predominantly associated with endosomal membranes. Within A549 cells, both Vps26A and Vps26B co-localized with actin-rich lamellipodia at the cell surface. These structures also co-localized with Vps35. Total internal reflection fluorescence microscopy confirmed the association of Vps26B with the plasma membrane in a stable HEK293 cell line expressing cyan fluorescent protein (CFP)-Vps26B. Based on these observations, we propose that the mammalian retromer complex is located at both endosomes and the plasma membrane in some cell types.
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