Lipids help epithelia stand tall

A nkyrin proteins organize the plasma membrane into specialized domains by binding to trans-membrane proteins and anchoring them to a submembranous network of spectrin and actin (1). Ankyrin-G, for example, helps build the axon initial segment of neurons and also promotes assembly of the lateral membrane of columnar epithelial cells (2). He et al. reveal that lipids play an essential role in targeting ankyrin-G and its binding partner , ␤II-spectrin, to the right location (3). " Ten years ago, we found that if you eliminate ankyrin-G from columnar epi-thelial cells you lose the entire lateral membrane, " says Vann Bennett from Duke University School of Medicine in Durham, NC. " The cells go from being 10 microns in height to being less than a micron. " ␤II-spec-trin also controls epithelial cell height (4), but how the two proteins promote lateral membrane assembly remains unclear. " We have to understand how they target to the lateral membrane, " says Bennett. " How does ankyrin-G know where to go? " Bennett and colleagues, led by grad student Meng He, recently discovered that ankyrin-G is palmitoylated and that blocking this lipid modifi cation by mutating a specifi c cysteine residue near the protein's N terminus prevented ankyrin-G from promoting lateral membrane assembly (5). He et al. now checked all 23 members of the DHHC family of palmitoyltrans-ferases and found that two of them—DHHC5 and DHHC8—localized to the lateral membrane of MDCK epithelial cells and palmi-toylated ankyrin-G (3). " Knocking down both of these enzymes eliminated ankyrin-G palm-itoylation and targeting to the lateral membrane , " says Bennett. As a result, the cells became shorter, just as they do in the complete absence of ankyrin-G. He et al. then turned their attention to ␤II-spectrin, which, in addition to binding ankyrin-G, can associate with membrane phospholipids through a phosphoinositide-binding pleckstrin homology domain. A ␤II-spectrin mutant unable to bind ankyrin-G lost its polarity, localizing to both the apical and lateral cell membranes. Surprisingly, however , a ␤II-spectrin mutant unable to bind phosphoinositides localized intracellularly, despite its continued ability to bind with high affi nity to ankyrin-G. Both mutants failed to promote lateral membrane assembly. " So it looks like this system requires palmitoylation of ankyrin-G, which, in turn, imposes polarity on ␤II-spectrin. But ␤II-spectrin requires both ankyrin and phosphoinositide binding [to localize specifically to the lateral mem-brane]. " ␤II-spectrin is thus a " coincidence detector " …