Lattices, rafts, and scaffolds: domain regulation of receptor signaling at the plasma membrane

Abstract

The plasma membrane is organized into various subdomains of clustered macromolecules. Such domains include adhesive structures (cellular synapses, substrate adhesions, and cell-cell junctions) and membrane invaginations (clathrin-coated pits and caveolae), as well as less well-defined domains such as lipid rafts and lectin-glycoprotein lattices. Domains are organized by specialized scaffold proteins including the intramembranous caveolins, which stabilize lipid raft domains, and the galectins, a family of animal lectins that cross-link glycoproteins forming molecular lattices. We review evidence that these heterogeneous microdomains interact to regulate substratum adhesion and cytokine receptor dynamics at the cell surface.

Figure 1.

Plasma membrane domains. (A) Depictions of lattices as multivalent interactions between pentameric Gal-3 and glycoproteins, rafts as a component of STALL by single particle tracking (Suzuki et al., 2007b), and scaffolds as a minimum oligomer of 15 Cav1 molecules. Illustrations are not drawn to scale. (B) Cav1 domains include caveolae, Cav1 scaffolds, and pY14Cav1 scaffold domains in focal adhesions. Cav1 scaffolds may function to restrict signaling of cytokine receptors, such as EGFR, through direct interactions. However, how stable Cav1 domains regulate dynamic raft behavior, both within the plane of the membrane and in the endocytosis of various ligands such as CT-b, remains unclear.

Figure 2.

Cav1 and Gal-3 in tumor progression. (1) In cells lacking the galectin lattice, Cav1 scaffolds recruit receptors and inhibit signaling. (2) Loss of Cav1 enables receptor signaling, promoting tumor cell proliferation and tumor growth. (3) Expression of Mgat5-branched N-glycans and Gal-3 and receptor recruitment to the galectin lattice limits receptor down-regulation via endocytosis, and promotes signaling potential. (4) Receptor sequestration by the galectin lattice overrides Cav1 suppressor activity, enabling elevated Cav1 expression in advanced tumors. Gal-3 and pY14Cav1 work together to promote focal adhesion turnover and enhance FAK activation, tumor cell migration, and metastasis. Competitive and coordinate action between Cav1 scaffolds and the galectin lattice at various stages of tumor progression may explain how Cav1 can act as both a tumor promoter and suppressor.