The Protein-tyrosine Phosphatase CD45 Reaches the Cell Surface via Golgi-dependent and -independent Pathways

Troy A Baldwin,Hanne L Ostergaard

doi:10.1074/jbc.m209075200

Abstract

CD45 is a receptor protein-tyrosine phosphatase essential for T cell development and lymphocyte activation. It is highly glycosylated, with multiple isoforms and glycoforms expressed on the cell surface depending on the cell type and stage of differentiation. Interestingly, we found two pools of newly synthesized CD45 expressed on plasma membrane, one of which arrived by 5 min after synthesis. The remaining pool of CD45 was fully glycosylated and began to arrive at the cell surface at approximately 15 min. The rapidly expressed population of CD45 possessed exclusively endoglycosidase H-sensitive N-linked carbohydrate. Additionally, this rapidly expressed pool of CD45 appeared on the cell surface in a brefeldin A (BFA)-insensitive manner, suggesting that it reached the cell surface independent of the Golgi complex. The remaining CD45 trafficked through the Golgi complex, and transport proceeded via a BFA-sensitive mechanism. These data suggest that CD45 is able to reach the cell surface via two distinct routes. The first is a conventional Golgi-dependent pathway that allows fully processed CD45 to be expressed. The second utilizes an ill defined mechanism that is independent of the Golgi, is BFA-resistant, and allows for the expression of CD45 with immature carbohydrate on the cell surface.

Highlights

The transmembrane protein-tyrosine phosphatase CD45 is required for both thymocyte development and T cell activation [1, 2]
Initial experiments performed to address the time required for newly synthesized CD45 to reach the plasma membrane indicated that a population of newly synthesized CD45 was able to traffic to the surface during a 5-min pulse at 37 °C
The amount of newly synthesized, higher molecular weight CD45 isolated from the cell surface increased through 60 min, whereas the amount of the lower molecular weight population remained fairly constant after 15 min

Summary

Introduction

The transmembrane protein-tyrosine phosphatase CD45 is required for both thymocyte development and T cell activation [1, 2]. Three alternatively spliced exons reside within the external domain of CD45 and contain numerous sites for potential Olinked carbohydrate additions These exons are developmentally regulated with respect to usage in T cells; and cells at different developmental stages have the potential to express vastly different forms of CD45. As a protein moves through the secretory pathway, a level of control is exerted at each stage of the transport process, from protein folding and vesicle budding at the ER to movement through the Golgi stacks and sorting at the trans-Golgi network (TGN) en route to its final destination [14] In many cases, this regulation is necessary for proper function of the protein. These data support the existence of a transport pathway where cargo can reach the cell surface extremely rapidly, without the requirement of the Golgi complex

Methods

Results

Conclusion