Mutational analysis of proprotein processing, subunit association, and shedding of the LAR transmembrane protein tyrosine phosphatase.

The LAR transmembrane protein tyrosine phosphatase (PTPase) is expressed on the cell surface as a complex of two noncovalently associated subunits derived from a proprotein. The 150-kDa E-subunit contains most of the extracellular region, including the immunoglobulin-like and fibronectin type-III-like domains, whereas the 85-kDa P-subunit contains a short ectodomain, the transmembrane peptide, and the two intracellular PTPase domains. The LAR extracellular region is released from the cell surface, suggesting that shedding may be a mechanism to regulate LAR PTPase function. Functional regions necessary for LAR proprotein processing, subunit association, and shedding were determined by analyzing the effect of amino acid substitutions of residues surrounding the cleavage site and scanning the P-subunit ectodomain. Three amino acid residues were identified, two within a penta-arginine sequence and one C-terminal to the cleavage site, that are essential for efficient LAR proprotein cleavage. Several noncontiguous amino acid residues were also identified that play an essential role in LAR subunit association. LAR shedding is shown to be a consequence of proteolytic cleavage at a second site within the P-subunit ectodomain near the transmembrane peptide.