P125, A COPII INTERACTING PROTEIN

Coat protein complex II (COPII) coated vesicles/carriers mediate the export of proteins from the ER via specialized domains known as the ER exit sites (ERES). p125A is a protein of 125 kDa that interacts with Sec23A, a component of the COPII coat. It was found to participate in the organization of ER exit sites (Tani et al., 1999; Shimoi et al., 2005). In this current study, using GST-pulldown and mass spectrometry analysis, p125A is also identified as one of the proteins interacting with Sec31A, another member of the COPII coat. Deletion mutant analysis of p125A narrowed its Sec31A-interacting domain to be within residues 260-600. Binding of p125A to Sec31A is direct and independent of the interactions with Sec23A. Gel filtration and immune-depletion studies suggest that p125A exists as a ternary complex with the Sec13/Sec31 heterotetramer in the cytosol. Silencing of p125A using siRNA caused the Golgi apparatus is fragmented. The export of proteins from the ER was also delayed in p125A silenced cells suggesting that p125A is essential for efficient protein trafficking in the early secretory pathway. p125A knockdown also afforded a decrease in membrane association for Sec13 and Sec31A, but not Sec23A. Knockdown of Sec31A results in a redistribution of p125A to the cytosol, indicating that Sec31A is important for the association of p125A to the membrane. In summary, the results suggest that p125A is involved in stabilizing the association of the Sec13/Sec31 heterotetramer to the ERES, which is essential for efficient ER export.

Author dc:creator

• ONG YAN SHAN

• p125, COPII, Sec31, Sec23, ER-Golgi, membrane trafficking