Out-of-the-groove transport of lipids by TMEM16 and GPCR scramblases.

Phospholipid scramblases externalize phosphatidylserine to facilitate numerous physiological processes. Several members of the structurally unrelated TMEM16 and G protein-coupled receptor (GPCR) protein families mediate phospholipid scrambling. The structure of a TMEM16 scramblase shows a membrane-exposed hydrophilic cavity, suggesting that scrambling occurs via the ‟credit-card" mechanism where lipid headgroups permeate through the cavity while their tails remain associated with the membrane core.

Known structures and unknown mechanisms of TMEM16 scramblases and channels.

The TMEM16 family of membrane proteins is composed of both Ca-gated Cl channels and Ca-dependent phospholipid scramblases. The functional diversity of TMEM16s underlies their involvement in numerous signal transduction pathways that connect changes in cytosolic Ca levels to cellular signaling networks. Indeed, defects in the function of several TMEM16s cause a variety of genetic disorders, highlighting their fundamental pathophysiological importance.

TMEM16 proteins: unknown structure and confusing functions.

The TMEM16 family of membrane proteins, also known as anoctamins, plays key roles in a variety of physiological functions that range from ion transport to phospholipid scrambling and to regulating other ion channels. The first two family members to be functionally characterized, TMEM16A (ANO1) and TMEM16B (ANO2), form Ca(2+)-activated Cl(-) channels and are important for transepithelial ion transport, olfaction, phototransduction, smooth muscle contraction, nociception, cell proliferation and control of neuronal excitability. The roles of other family members, such as TMEM16C (ANO3), TMEM16D (ANO4), TMEM16F (ANO6), TMEM16G (ANO7) and TMEM16J (ANO9), remain poorly understood and controversial.

Ca2+-dependent phospholipid scrambling by a reconstituted TMEM16 ion channel.

Phospholipid (PL) scramblases disrupt the lipid asymmetry of the plasma membrane, externalizing phosphatidylserine to trigger blood coagulation and mark apoptotic cells. Recently, members of the TMEM16 family of Ca(2+)-gated channels have been shown to be involved in Ca(2+)-dependent scrambling. It is however controversial whether they are scramblases or channels regulating scrambling.

Proton block of the CLC-5 Cl-/H+ exchanger.

CLC-5 is a H(+)/Cl(-) exchanger that is expressed primarily in endosomes but can traffic to the plasma membrane in overexpression systems. Mutations altering the expression or function of CLC-5 lead to Dent's disease. Currents mediated by this transporter show extreme outward rectification and are inhibited by acidic extracellular pH.

Basis of substrate binding and conservation of selectivity in the CLC family of channels and transporters.

Ion binding to secondary active transporters triggers a cascade of conformational rearrangements resulting in substrate translocation across cellular membranes. Despite the fundamental role of this step, direct measurements of binding to transporters are rare. We investigated ion binding and selectivity in CLC-ec1, a H(+)-Cl(-) exchanger of the CLC family of channels and transporters.