Actin filaments are major components of at least 15 distinct structures in metazoan cells. These filaments assemble from a common pool of actin monomers, but do so at different times and places, and in response to different stimuli. All of these structures require actin-filament assembly factors. To date, many assembly factors have been identified, including Arp2/3 complex, multiple formin isoforms and spire. Now, a major task is to figure out which factors assemble which actin-based structures. Here, we focus on structures at the plasma membrane, including both sheet-like protrusive structures (such as lamellipodia and ruffles) and finger-like protrusions (such as filopodia and microvilli). Insights gained from studies of adherens junctions and the immunological synapse are also considered.

Formin proteins modulate both nucleation and elongation of actin filaments through processive movement of their dimeric formin homology 2 (FH2) domains with filament barbed ends. Mammals possess at least 15 formin genes. A subset of formins termed "diaphanous formins" are regulated by autoinhibition through interaction between an N-terminal diaphanous inhibitory domain (DID) and a C-terminal diaphanous autoregulatory domain (DAD). Here, we found several striking features for the mouse formin, INF2. First, INF2 interacted directly with actin through a region C-terminal to the FH2. This second interacting region sequesters actin monomers, an activity that is dependent on a WASP homology 2 (WH2) motif. Second, the combination of the FH2 and C-terminal regions of INF2 resulted in its curious ability to accelerate both polymerization and depolymerization of actin filaments. The mechanism of the depolymerization activity, which is novel for formin proteins, involves both the monomer binding ability of the WH2 and a potent severing activity that is dependent on covalent attachment of the FH2 to the C terminus. Phosphate inhibits both the depolymerization and severing activities of INF2, suggesting that phosphate release from actin subunits in the filament is a trigger for depolymerization. Third, INF2 contains an N-terminal DID, and the WH2 motif likely doubles as a DAD in an autoinhibitory interaction.Formins are actin assembly factors for a wide variety of actinbased structures, including cytokinetic rings, certain types of stress fiber, filopodia, actin filaments around endosomes, and yeast actin cables (1) (reviewed in Refs. 2 and 3). To date, all direct interaction with actin occurs through the formin homology 2 (FH2) 2 domain, which is dimeric and binds at filament barbed ends. All FH2 domains studied possess the following activities: 1) acceleration of filament nucleation; 2) processive movement with the elongating filament barbed end, which can influence both elongation and depolymerization rate at this end; and 3) the ability to block barbed end capping by capping proteins such as gelsolin and heterodimeric capping protein.The FH1 domain, N-terminal to the FH2, binds profilin and accelerates elongation from FH2-bound barbed ends in a profilin-dependent manner (4, 5).Although individual formins differ quantitatively in some activities, the overall effect of formins is to enhance actin assembly rate. In addition, some FH2 domains can bundle filaments (6 -8). No actin binding ability has been identified outside the FH2 domain.Most eukaryotes possess multiple formins, with mammals having at least 15 formin genes that segregate into seven classes based on FH2 domain phylogenetic analysis (9). One exclusively metazoan group, termed inverted formins (INFs), were thought to be unique in the placement of their FH1 and FH2 domains at the N terminus of the protein, as opposed to all other formins, which have additional domains N-terminal to the FH1. This difference has regulatory significance, because the N termini of ma...

In addition to its ability to accelerate filament assembly, which is common to formins, INF2 is a formin protein with the unique biochemical ability to accelerate actin filament depolymerization. The depolymerization activity of INF2 requires its actin monomer-binding WASP homology 2 (WH2) motif. In this study, we show that INF2 is peripherally bound to the cytoplasmic face of the endoplasmic reticulum (ER) in Swiss 3T3 cells. Both endogenous INF2 and GFP-fusion constructs display ER localization. INF2 is post-translationally modified by a C-terminal farnesyl group, and this modification is required for ER interaction. However, farnesylation is not sufficient for ER association, and membrane extraction experiments suggest that ionic interactions are also important. The WH2 motif also serves as a diaphanous autoregulatory domain (DAD), which binds to the N-terminal diaphanous inhibitory domain (DID), with an apparent dissociation constant of 1.1 μM. Surprisingly, the DID-DAD interaction does not inhibit the actin nucleation activity of INF2; however, it does inhibit the depolymerization activity. Point mutations to the DAD/WH2 inhibit both the DID-DAD interaction and depolymerization activity. Expression of GFP-INF2 containing these DAD/WH2 mutations causes the ER to collapse around the nucleus, with accumulation of actin filaments around the collapsed ER. This study is the first to show the association of an actin-assembly factor with the ER.

D assemblies make up half of the von Willebrand factor (VWF), yet are of unknown structure. D1 and D2 in the prodomain and D′D3 in mature VWF at Golgi pH form helical VWF tubules in Weibel Palade bodies and template dimerization of D3 through disulfides to form ultralong VWF concatemers. D′D3 forms the binding site for factor VIII. The crystal structure of monomeric D′D3 with cysteine residues required for dimerization mutated to alanine was determined at an endoplasmic reticulum (ER)-like pH. The smaller C8-3, TIL3 (trypsin inhibitor-like 3), and E3 modules pack through specific interfaces as they wind around the larger, N-terminal, Ca2+-binding von Willebrand D domain (VWD) 3 module to form a wedge shape. D′ with its TIL′ and E′ modules projects away from D3. The 2 mutated cysteines implicated in D3 dimerization are buried, providing a mechanism for protecting them against premature disulfide linkage in the ER, where intrachain disulfide linkages are formed. D3 dimerization requires co-association with D1 and D2, Ca2+, and Golgi-like acidic pH. Associated structural rearrangements in the C8-3 and TIL3 modules are required to expose cysteine residues for disulfide linkage. Our structure provides insight into many von Willebrand disease mutations, including those that diminish factor VIII binding, which suggest that factor VIII binds not only to the N-terminal TIL′ domain of D′ distal from D3 but also extends across 1 side of D3. The organizing principle for the D3 assembly has implications for other D assemblies and the construction of higher-order, disulfide-linked assemblies in the Golgi in both VWF and mucins.

Tau protein is present in six different splice forms in the human brain and interacts with microtubules via either 3 or 4 microtubule binding repeats. An increased ratio of 3 repeat to 4 repeat isoforms is associated with neurodegeneration in inherited forms of frontotemporal dementia. Tau overexpression diminishes axonal transport in several systems, but differential effects of 3 repeat and 4 repeat isoforms have not been studied. We examined the effects of tau on mitochondrial transport and found that both 3 repeat and 4 repeat tau change normal mitochondrial distribution within the cell body and reduce mitochondrial localization to axons; 4 repeat tau has a greater effect than 3 repeat tau. Further, we observed that the 3 repeat and 4 repeat tau cause different alterations in retrograde and anterograde transport dynamics with 3 repeat tau having a slightly stronger effect on axon transport dynamics. Our results indicate that tau-induced changes in axonal transport may be an underlying theme in neurodegenerative diseases associated with isoform specific changes in tau's interaction with microtubules. KeywordsAlzheimer's disease; axonal transport; mitochondria; tau; tauopathy In the adult human brain, there are six isoforms of the tau gene expressed, with an approximately 50/50 ratio of isoforms containing either 3 microtubule binding repeats (not including exon 10) or 4 microtubule binding repeats (including exon 10) (Goedert et al. 1989;Kosik et al. 1989;Goedert and Jakes 1990). Although it has been postulated that the 4 Figure S1. Similar 3R and 4R tau over-expression levels in cortical neurons. As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peerreviewed and may be re-organized for online delivery, but are not copy-edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors. NIH Public Access