The MAM (Meprin/A5-protein/PTPmu) Domain Is a Homophilic Binding Site Promoting the Lateral Dimerization of Receptor-like Protein-tyrosine Phosphatase μ

Abstract

The MAM (meprin/A5-protein/PTPmu) domain is present in numerous proteins with diverse functions. PTP belongs to the MAM-containing subclass of protein-tyrosine phosphatases (PTP) able to promote cell-to-cell adhesion. Here we provide experimental evidence that the MAM domain is a homophilic binding site of PTP. We demonstrate that the MAM domain forms oligomers in solution and binds to the PTP ectodomain at the cell surface. The presence of two disulfide bridges in the MAM molecule was evidenced and their integrity was found to be essential for MAM homophilic interaction. Our data also indicate that PTP ectodomain forms oligomers and mediates the cellular adhesion, even in the absence of MAM domain homophilic binding. Reciprocally, MAM is able to interact homophilically in the absence of ectodomain trans binding. The MAM domain therefore contains independent cis and trans interaction sites and we predict that its main role is to promote lateral dimerization of PTP at the cell surface. This finding contributes to the understanding of the signal transduction mechanism in MAM-containing PTPs.The phosphorylation state of numerous signaling proteins is controlled by opposing activities of protein-tyrosine kinases and protein-tyrosine phosphatases (PTP) 1 (1). The family of PTPs consists of soluble and receptor-like PTPs (RPTPs) (2). Whereas the intracellular region of RPTPs is relatively similar in all representatives containing either a single or two PTP domains, the extracellular region has a large diversity. PTP belongs to subclass IIB, called "MAM-containing PTP" (2). Besides the MAM domain (meprin/A5-protein/PTPmu domain; Ref.3), their extracellular region contains a single immunoglobulin (Ig)-like domain and four fibronectin (FN) III repeats (4). This structural architecture of ectodomain is similar to members of the cell-adhesion molecule superfamily.PTP is strongly expressed in the endothelial cell layer of the arteries and continuous capillaries as well as in cardiac muscle, bronchial and lung epithelia, retina, and several brain areas (4 -6). At the subcellular level, it is localized at sites of cell-cell contact (7). In this regard, it has been demonstrated that PTP restores E-cadherin-mediated cellular adhesion, when it is expressed in LNCaP human prostate carcinoma cells (8). Physiologically, PTP has been shown to be involved in promotion and regulation of neurite outgrowth (5, 9).Numerous experiments have clearly demonstrated that the extracellular region of PTP promotes cell-cell aggregation in a Ca 2ϩ -independent manner (10, 11). The homophilic binding has been also evidenced in the ectodomains of PTP (12) and PTP (13), strongly suggesting that these RPTPs may be involved in signal transduction through cell-to-cell contact in vivo. Evidence concerning the physiological role of PTP-mediated homophilic binding has been reported in a recent article (14) showing that homophilic interactions trigger rearrangements of the axonal growth cone. However, the molecular mechanism of this interaction remains larg...

MDGA1, an IgSF molecule containing a MAM domain, heterophilically associates with axon- and muscle-associated binding partners through distinct structural domains

“…It is also plausible that MDGA1 is required for later path-finding and targeting processes, but not at the onset of its expression. and mediates lateral (cis) homophilic interactions in neuropilin-1 (Nakamura et al, 1998) and PTPµ (Cismasiu et al, 2004). Our experiments do not exclude the potential of homophilic association among MDGA1 molecules, that might not be detected by the overlay assay used here.…”

Intersubunit and Domain Interactions of the Meprin B Metalloproteinase

“…Although there is evidence that the intradomain disulfide bridges in MAM domains in proteins such as protein-tyrosine phosphatase are essential for MAM homophilic interactions, there is no evidence for interdomain disulfide bridging in MAM domains in proteins other than meprins (12). The finding herein that the ␤C274A mutant has full enzymatic activity and results in the formation of a noncovalent meprin dimer indicates that the meprin ␤ subunit has a dimerization interface that is not dependent on intersubunit disulfide bridges.…”

“…1). The MAM domain is present in many cell surface proteins and is thought to be involved in cell-cell adhesion, protein-protein interactions, and signal transduction (12). The TRAF domain is found in a diverse population of intracellular proteins and is also involved in protein associations and signal transduction; meprins are the only plasma membrane and extracellular proteins known to contain TRAF domains (13).…”

Expression of Human MDGA1 Increases Cell Motility and Cell-Cell Adhesion and Reduces Adhesion to Extracellular Matrix Proteins in MDCK Cells

“…Several MAM domain-containing proteins later identified, including zonadhesin [19], nephronectin [20], POEM [21], and neuropilins [22], have been shown to be involved in different aspects of cell adhesion and migration. It has been reported that the MAM domain mediates lateral (cis) homophilic interactions in PTP μ and κ [15,23] and neuropilin-1 [24,25]. In the developing chicken nervous system MDGA1 heterophilically interacts with axon-rich regions mainly through its MAM domain, and with differentiating muscle through its Ig-repeat-containing Nterminal region [4].…”

“…Some of the GPIlinked IgSF proteins are involved in a variety of specific cell-cell interactions and/or in migration, such as LAMP, BIG-1, neurotrimin, CEPU-1, GP55 [7][8][9], CEA, CEACAM-6, NCAM p120 [10][11][12], F3/F11/contactin and TAG1/axonin-1 [13,14]. In addition, the presence of the MAM and/or Ig domains confers to these proteins the capacity to interact with other cells through homophilic and/or heterophilic interactions [2,4,15].…”