The MAL proteolipid is a component of the detergent-insoluble membrane subdomains of human T-lymphocytes

Abstract

The human mal gene, identified during a search for cDNAs selectively expressed during T-cell development, encodes a highly hydrophobic protein belonging to a group of proteins, termed proteolipids, characterized by their unusual property of being soluble in organic solvents used to extract cell lipids. To study the localization of the MAL protein we have prepared stable transfectants expressing the MAL protein tagged with a c-myc epitope (MAL/c-myc) using human epithelial A-498 cells. Immunofluorescence analysis suggested that MAL/c-myc is localized mainly to cholesterol-enriched structures with a post-Golgi location and, at low levels, in early endosomes. Moreover, extraction of A-498 cell membranes with Triton X-100 (TX100) and fractionation by centrifugation to equilibrium in sucrose gradients demonstrated the presence of MAL/c-myc in the detergent-insoluble buoyant fraction, known to be enriched in glycolipids and cholesterol. To compare the behaviour of MAL in T-cells with that in epithelial A-498 cells, we prepared stably transfected cells expressing MAL/c-myc using human Jurkat T-cells. When TX100 extracts from Jurkat cells were subjected to centrifugation to equilibrium in sucrose gradients we found MAL exclusively in the floating fractions, together with molecules characteristic of the T-cell insoluble complexes, such as the tyrosine kinase p56lck, the glycosylphosphatidylinositol-anchored protein CD59 and the ganglioside GM1. These results, taken together, indicate that the MAL proteolipid is a component of the detergent-resistant membrane microdomains present in T-lymphocytes, and suggest that MAL might play a role in modulating the function of these microdomains during T-cell differentiation.

MAL gene expression in esophageal cancer suppresses motility, invasion and tumorigenicity and enhances apoptosis through the Fas pathway

“…The Mal protein is known to be a proteolipid component of the detergent-resistant membrane microdomains or lipid rafts in a restricted pattern of cell types including polarized epithelial cells, myelinforming cells and T lymphocytes. Mal appears to act as machinery for raft remodeling during differentiation in T cell and for apical transport in epithelial cells (Millan et al, 1997). There was no previous evidence of a correlation between MAL expression and human epithelial malignancies.…”

Recombinant Expression of the MAL Proteolipid, a Component of Glycolipid-enriched Membrane Microdomains, Induces the Formation of Vesicular Structures in Insect Cells

“…Cells-The MAL proteolipid has been found as a component of detergent-insoluble membranes in MDCK cells (11), oligodendrocytes (12), and T lymphocytes (14). Transient expression of the MAL protein in mammalian cells produces the accumulation of the MAL proteolipid in large vesicular structures containing membranes resistant to detergent solubilization (11,14).…”

“…Transient expression of the MAL protein in mammalian cells produces the accumulation of the MAL proteolipid in large vesicular structures containing membranes resistant to detergent solubilization (11,14). We adopted a baculovirus system to express the MAL protein tagged at its COOH terminus with a c-Myc epitope and a polyhistidine tag in Sf21 insect cells.…”

“…Transient expression of MAL in COS-7 cells, as in other cell lines (11,14), produces the accumulation of MAL in large vesicles. In this work, we have adopted the baculovirus expression system to study the possible vesiculation induced by MAL expression.…”

“…Thus, MAL gene expression is both tissue-and differentiation-specific and appears to be modulated by elements distal to its 5Ј-proximal promoter region (13). In all of the cell types in which is expressed, MAL has been identified as a component of detergentinsoluble membrane microdomains (11,12,14). This fact, the identification of MAL in TGN-related vesicles in epithelial cells (14), and its predominance in apical transport vesicles in MDCK cells (11) have led to the proposal that MAL is a component of the protein sorting or vesiculation machinery of the glycolipid-mediated pathway of transport (11,14).…”