The Sec23p-Sec24p complex is a component of coat protein II-coated vesicles involved in protein export from the endoplasmic reticulum. We previously identified a novel Sec23p-interacting protein, p125, which consists of 1000 amino acids and comprises a proline-rich region and a phospholipase A, homology region. p125, when ectopically expressed in cultured cells, localizes to endoplasmic reticulum-Golgi intermediate regions. In the present study we showed that expressed p125 principally colocalizes with p115 and GM130, both of which are involved in vesicle tethering to Golgi membranes. Next, we determined the functional regions of p125 by expressing a p125 series with deletions. The results showed that the proline-rich region (residues 135-259) is responsible for the binding to Sec23p. For the correct localization of p125, a region (residues 135-1000) comprising both the proline-rich and phospholipase A, homology regions was required.