Multimerisation of receptor-type protein tyrosine phosphatases PTPBR7 and PTP-SL attenuates enzymatic activity

Abstract

Dimerisation of receptor-type protein tyrosine phosphatases (RPTPs) represents an appealing mechanism to regulate their enzymatic activity. Studies thus far mostly concern the dimerisation behaviour of RPTPs possessing two tandemly oriented catalytic PTP domains. Mouse gene Ptprr encodes four different protein isoforms (i.e. PTPBR7, PTP-SL and PTPPBSgamma-42/37) that contain a single PTP domain. Using selective membrane permeabilisation we here demonstrate that PTP-SL, like PTPBR7, is a single membrane-spanning RPTP. Furthermore, these two receptor-type PTPs constitutively formed homo- and hetero-meric complexes as witnessed in chemical cross-linking and co-immunoprecipitation experiments, in sharp contrast to the cytosolic PTPPBSgamma-42 and PTPPBSgamma-37 PTPRR isoforms. This multimerisation occurs independently of the PTP domain and requires the transmembrane domain and/or the proximal hydrophobic region. Using overexpression of a PTPBR7 mutant that essentially lacks the intracellular PTP domain-containing segment, a monomer-mimicking state was forced upon full-length PTPBR7 immunoprecipitates. This resulted in a significant increase in the enzymatic activity of the PTPRR PTP domain, which strengthens the notion that multimerisation represents a general mechanism to tone down RPTP catalytic activity.

Follicle-Stimulating Hormone (FSH)-dependent Regulation of Extracellular Regulated Kinase (ERK) Phosphorylation by the Mitogen-activated Protein (MAP) Kinase Phosphatase MKP3

“…As we show below, the pan-PTP inhibitor Na 2 VO 3 (19,20), which should inhibit PTP-SL (15), does not block FSH-stimulated ERK phosphorylation. PTP-SL has a molecular weight of ϳ55 kDa, not 100 kDa (21). A second antibody directed against PTPBR7, another phosphatase sharing protein sequence homology with PTP-SL except for a 127-amino acid insertion in the N terminus (14), does not detect a signal on Western blotting analyses of GC extracts at 100 kDa (10).…”

PTPRR (protein tyrosine phosphatase, receptor type, R)

“…Depending on the translational start site choice, the other isoforms produce either PTP-SL-like vesicle-associated versions or, more likely, cytosolic proteins that resemble murine PTPPBSγ. Intriguingly, the mouse transmembrane PTPRR isoforms form multimeric complexes and their relative PTP activity is significantly lower than that of the cytosolic PTPPBSγ type (Noordman et al, 2008). This predicts regulatory potential for any biomolecule that would bind to the PTPBR7 and/or PTP-SL parts that are N-terminal of the transmembrane segment.…”

“…No detailed studies on the subcellular localization of human PTPRR protein isoforms have been performed and, again, only inference from mouse and rat data (Shiozuka et al, 1995;Ogata et al, 1999;Van Den Maagdenberg et al, 1999;Dilaver et al, 2003;Noordman et al, 2006;Noordman et al, 2008;Hendriks et al, 2009) remains. Based on this, it seems reasonable to postulate that isoform #1 will display a PTPBR7-like cell surface expression and that isoform #3 may reside on the trans-Golgi network and multivesicular bodies or sorting endosomes, like mouse PTP-SL does.…”

“…In that case, the PTPPBSβ open reading frame would be 30 nucleotides shorter, and a 535 residue-spanning protein of 60 kDa would be synthesised. Although isoform #3 lacks an obvious signal peptide preceding the transmembrane segment, one may expect that like PTP-SL (Noordman et al, 2008) it will behave as a type III transmembrane molecule (Spiess, 1995). Human PTPRR transcript type #4 displays an open reading frame that predicts the synthesis of a 451 aa PTPRR isoform, coined PTPPBSδ (Augustine et al, 2000), that has a unique three amino acid N-terminus before it proceeds with the exon 5-encoded PTPRR read.…”

“…The transcript #5 and #6 derived types will lack all PTP domain residues and also the KIM segment is far from complete, all due to a reading frame change that is caused by exon 7 skipping. Depending on the AUG choice, a small protein that would contain the TM domain might be produced, which could theoretically influence the multimerization behaviour of the PTPBR7 and PTP-SL type isoforms (Noordman et al, 2008). PTPRR splice forms that manage to maintain the long version of exon 12 (e.g.…”