MSAP Is a Novel MIR-interacting Protein That Enhances Neurite Outgrowth and Increases Myosin Regulatory Light Chain

Abstract

Interactions between the cell membrane and the cytoskeleton play a major role in different aspects of cell differentiation, such as cell motility, cell division, and establishment of cellular architecture. Of particular importance in this context is the association of cortical actin with the cell membrane. The ERM 1 proteins, ezrin, radixin, and moesin, are members of the large 4.1 protein family and are involved in membrane-cytoskeleton interactions. They link the actin cytoskeleton to membranebound proteins located at different membrane sites, i.e. microvilli, membrane ruffles, and cell-cell contacts (1-3). This is accomplished by binding of the C-terminal part of the ERM proteins to actin and of the N-terminal FERM domain to specific membrane proteins (4). FERM domains have been found in many different proteins and are thought to be modules for protein-protein and protein-membrane interactions (5). The ERM proteins are involved in cell adhesion and signal transduction events through phosphorylation (6, 7) and interaction with phosphoinositides (8). The ERM proteins play an important role in the activation of Rho family proteins and can interact both downstream (9) and upstream (10) of Rho.In addition, the ERM proteins are involved in membrane dynamics (11). When ezrin, radixin, and moesin were simultaneously inactivated by antisense treatment in epithelial cells, cell-cell and cell-substratum adhesion was altered (12). Double suppression of radixin and moesin, but not ezrin and radixin or moesin, alters growth cone motility, inhibiting neurite extension (13). In contrast, overexpression of ezrin in insect cells leads to enhanced cell adhesion (14). The mechanisms behind these effects are, however, not well understood, but ERM proteins are known to interact with various proteins such as CD44 and ICAM-1, -2, and -3, which helps in establishing membrane specializations (for review, see Ref. 7). The identification and characterization of further binding partners for ERM proteins can give new insights into the function of these proteins.We have recently identified a novel ERM family protein, MIR, which has an ERM domain at the N terminus and lacks actin binding, instead possessing a RING domain in the Cterminal region (15). Overexpression of MIR abrogated neurite outgrowth in PC12 cells, an effect that may be brought about by its interaction with the myosin regulatory light chain (MRLC). We describe here a novel protein interacting with MIR, called MIR-interacting saposin-like protein (MSAP), which stimulates neurite outgrowth. Sequence comparisons showed that MSAP contains a saposin domain, which is found, among others, in the sphingolipid activator proteins, the saposins (16), and in some saposin-like proteins with a similar structure (17). The effects of MSAP on neurite outgrowth were reduced by MIR. MIR was shown to induce a decrease in the levels of MRLC, which could be blocked by overexpression of MSAP or by inhibition of proteasome activity. Evidence was obtained that the decrease in MRLC levels by MIR invo...

Identification of targets of the Wnt pathway destruction complex in addition to β-catenin

“…3), raising the possibility of coordinated regulation of certain cellular processes. One exciting group consists of the cytoskeletal regulatory proteins myosin regulatory light chain (MRLC) and transmembrane protein 4 (TMEM4), which are known to interact with each other (32). Both appear to be specifically regulated by the Wnt signaling pathway destruction complex, raising the possibility that cell motility can be developmentally controlled by canonical Wnt signaling.…”

Hepatic lipase maturation: a partial proteome of interacting factors

“…Thus, in any of these capacities, TorsinB is a candidate for HL maturation; however, without detecting this protein in untreated cells, its participation in normal HL maturation is tentative. MSAP prevents ubiquitination of myosin regulatory light chain, an important component of the cytoskeleton (48). However, a function in HL maturation is unlikely, and its detection may result from the presence of tubulin/actin in HL-TAP isolates.…”

The Relevance of Structural Biology in Studying Molecules Involved in Parasite–Host Interactions: Potential for Designing New Interventions

“…[44] Due to the many variations and adaptations in protein families such as this, experimental three-dimensional structures of individual members are indispensable. Typical structural characteristics of the Sap domain include a jaw-like arrangement of a four-or five-helix bundle where a kink in the third helix can lead to occurrence of two helices (see Figure 1).…”

“…Despite the conservation of the Sap domain, the fold of individual SAPLIPs is adapted to carry out particular and diverse functions at biological membranes ranging from cytolytic properties of amoebapores, [28] granulysin [41] and NK-lysin [42] to plant aspartic proteases [43] and neurotrophic factors. [44] Due to the many variations and adaptations in protein families such as this, experimental three-dimensional structures of individual members are indispensable. Typical structural characteristics of the Sap domain include a jaw-like arrangement of a four-or five-helix bundle where a kink in the third helix can lead to occurrence of two helices (see Figure 1).…”

“…In this model, each monomer is bound to the membrane surface via a highly positively charged surface area, and the high density of positive charges on the SAPLIPs is assumed to cause membrane destabilisation by electroporation. [46] Noteworthy is the observation that the crystal structure of Na-SLP-1 from human hookworm and the membrane binding data obtained for this protein [44] is similar to granulysin; however, neither haemolytic nor liposome destabilisation has been observed thus far for Na-SLP-1.(2) For human saposins C and D, a mechanism has been proposed that comprises the annealing of dimeric species to the membrane via the amphipathic helices α2 and α3-4, respectively. This 4 Mason et al…”