Even though plasma membranes are fluid with the viscosity of a light olive oil, they are organized into discrete neighborhoods or compartments with specialized assemblies of membrane-spanning proteins. Our laboratory has discovered an adaptable mechanism based on ankyrin adaptors and their spectrin partners that coordinates functionally related membrane-spanning proteins within micron-scale domains in diverse vertebrate plasma membranes ranging from erythrocytes to excitable membranes in the nervous system and heart (Bennett, V. and Lorenzo, D.N. 2016 Curr Top Membr. 77:143-84). Ankyrins recognize cytoplasmic domains of membrane transporters and cell adhesion proteins (15 protein families identified so far) through independently evolved interactions of intrinsically disordered sequences with a highly conserved peptide-binding groove formed by the ANK repeat solenoid. Ankyrins are coupled to spectrins, which are elongated organelle-sized proteins that form mechanically resilient arrays through cross-linking by specialized actin filaments capped with adducin (Bennett and Lorenzo, 2016).
A partnership between spectrin, ankyrin, and cell adhesion molecules first emerged in bilaterians over 500 million years ago (Bennett and Lorenzo, 2016). The basic bilaterian spectrin-ankyrin toolkit markedly expanded in vertebrates through gene duplications combined with variation in unstructured intramolecular regulatory sequences as well as independent evolution of ankyrin-binding activity by ion transporters involved in action potentials and calcium homeostasis (Bennett and Lorenzo, 2016). In addition, giant vertebrate ankyrins with specialized roles in axons acquired new coding sequences by exon shuffling (Jenkins, P.M. et al. 2015 Proc Natl Acad Sci. 112:957-64; Tseng, W.C. et al. 2015 Proc Natl Acad Sci. 112:1214-9). We speculate that early axon initial segments and epithelial lateral membranes initially were based on spectrin-ankyrin-cell adhesion molecule assemblies and subsequently served as “incubators”, where ion transporters independently acquired ankyrin-binding activity through positive selection.
In addition to protein interactions, we have recently discovered that assembly of spectrin/ankyrin in plasma membrane domains critically depends on palmitoylation of ankyrin-G by DHHC 5/8 palmitoyltransferases, as well as interaction of beta-2 spectin with phosphoinositide lipids (He, M. et al., 2014 J Cell Biol. 206:273-88). We also have found that that spectrin and ankyrin-G colocalize with DHHC 5/8 palmitoyltransferases in newly resolved microdomains on lateral membranes of columnar epithelial cells (He, M. et al., 2014). These spectrin/ankyrin microdomains are locally dynamic and determine membrane identity through opposing endocytosis of bulk lipids as well as specific proteins (Jenkins, P.M. et al. 2015 Science Advances 1:e1500301).
We have recently discovered unexpected intracellular roles for ankyrin in addition to functions at the plasma membrane (Bennett and Lorenzo, 2016). Ankyrin-B associates with organelles as the result of autoinhibition of association with plasma membrane proteins that is mediated by intramolecular association of a linker peptide with the ANK repeat solenoid. This linker peptide is encoded by a single exon that appeared early in vertebrate evolution following the whole genome duplication event where ankyrin-B and ankyrin-G diverged from a common ancestor. We have found that ankyrin-B is the principal membrane adaptor for the dynactin complex and couples dynactin to diverse organelles through binding of ankyrin-B to PI3P phosphoinositide lipids (Lorenzo, D.N. et al. 2014 J Cell Biol. 207:735-52).
We have recently discovered that human variants of ankyrin-B are relatively common and are linked to a new metabolic syndrome that is characterized by insulin insufficiency combined with age-dependent adiposity and insulin resistance (Lorenzo, D.N. et al., 2015). A core mechanism underlying adiposity in ankyrin-B metabolic syndrome is retention of plasma membrane Glut4 transporters in adipose tissue and skeletal muscle, resulting in a cell-autonomous increase in accumulation of lipid stores (Lorenzo et al. 2015 J Clin Invest. 125:3087-102).
As outlined above, we have resolved an extensive interactome centered on ankyrin and spectrin with functions both in long-range organization of plasma membranes and in organelle transport. Future research will focus on building on this molecular foundation to develop new insights into vertebrate physiology and human disease. We will address the questions related to the following topics:
1. Structural plasticity of axon initial segments: molecular basis and possible roles in adaptive responses of vertebrate nervous systems.
2. Ankyrin-B-dependent endocytosis of Glut4 transporters: cellular mechanism and possible roles adult onset adiposity.
3. Function of giant ankyrin-B in neurodevelopment and how mutation of ANK2 (the gene encoding ankyrin-B) leads to autism spectrum disorder.
This research has been supported by funding from the Howard Hughes Medical Institute, National Institutes of Health, and Muscular Dystrophy Association.
As of April 13, 2016