Structural analyses of FERM domain-mediated membrane localization of FARP1.

FARP1 is a multi-domain protein that is involved in regulating neuronal development through interacting with cell surface proteins such as class A Plexins and SynCAM 1. The N-terminal FERM domain in FARP1 is known to both promote membrane localization and mediate these protein interactions, for which the underlying molecular mechanisms remain unclear. Here we determined the crystal structures of the FERM domain of FARP1 from zebrafish, and those of FARP2 (a close homolog of FARP1) from mouse and zebrafish. These FERM domains adopt the three-leaved clover fold that is typical of all FERM domains. Our structures reveal a positively charged surface patch that is highly conserved in the FERM domain of FARP1 and FARP2. In vitro lipid-binding experiments showed that the FARP1 FERM domain binds specifically to several types of phospholipid, which is dependent on the positively charged surface patch. We further determined through cell-based analyses that this surface patch on the FERM domain underlies the localization of FARP1 to the plasma membrane, and that FERM domain interactions recruit it to postsynaptic sites in neurons.

Regulation of tankyrase activity by a catalytic domain dimer interface.

Tankyrases (TNKS and TNKS2) are enzymes that catalyze poly-ADP-ribosylation (PARsylation) of their target proteins. Tankyrase-mediated PARsylation plays critical regulatory roles in cell signaling, particularly in the Wnt/ß-catenin pathway. The sterile alpha motif (SAM) domain in tankyrases mediates their oligomerization, which is essential for tankyrase function. The oligomerization regulates the catalytic activity of tankyrases, but the underlying mechanism is unclear. Our analyses of crystal structures of the tankyrase catalytic domain suggest that formation of a head-to-head dimer regulates the catalytic activity. Our activity assays show that residues in the catalytic domain dimer interface are important for the PARsylation activity of tankyrases both in solution and cells. The dimer is weak and may only form in the context of the SAM domain-mediated oligomers of tankyrases, consistent with the dependence of the tankyrase activity on the SAM domain.

Class A Plexins Are Organized as Preformed Inactive Dimers on the Cell Surface.

Plexins are single-pass transmembrane receptors that bind the axon guidance molecules semaphorins. Single-pass transmembrane proteins are an important class of receptors that display a wide variety of activation mechanisms, often involving ligand-dependent dimerization or conformational changes. Resolving the activation mechanism and dimerization state of these receptors is extremely challenging, especially in a live-cell environment. Here, we report on the dimerization state of PlexinA4 and its response to activation by semaphorin binding. Semaphorins are dimeric molecules that activate plexin by binding two copies of plexin simultaneously and inducing formation of a specific active dimer of plexin. An open question is whether there are preexisting plexin dimers that could act as autoinhibitory complexes. We address these questions with pulsed interleaved excitation fluorescence cross-correlation spectroscopy (PIE-FCCS). PIE-FCCS is a two-color fluorescence microscopy method that is directly sensitive to protein dimerization in a live-cell environment. With PIE-FCCS, we show that inactive PlexinA4 is dimerized in the live-cell plasma membrane. By comparing the cross correlation of full-length PlexinA4 to control proteins and plexin mutants, we show that dimerization of inactive PlexinA4 requires the Sema domain, but not the cytoplasmic domain. Ligand stimulation with Sema6A does not change the degree of cross correlation, indicating that plexin activation does not lead to higher-order oligomerization. Together, the results suggest that semaphorin activates plexin by disrupting an inhibitory plexin dimer and inducing the active dimer.