Characterization of a mutant profilin with reduced actin-binding capacity: effects in vitro and in vivo

Structural basis for mutation-induced destabilization of profilin 1 in ALS

Mutations in profilin 1 (PFN1) are associated with amyotrophic lateral sclerosis (ALS); however, the pathological mechanism of PFN1 in this fatal disease is unknown. We demonstrate that ALS-linked mutations severely destabilize the native conformation of PFN1 in vitro and cause accelerated turnover of the PFN1 protein in cells. This mutation-induced destabilization can account for the high propensity of ALS-linked variants to aggregate and also provides rationale for their reported loss-of-function phenotypes in cell-based assays. The source of this destabilization is illuminated by the X-ray crystal structures of several PFN1 proteins, revealing an expanded cavity near the protein core of the destabilized M114T variant. In contrast, the E117G mutation only modestly perturbs the structure and stability of PFN1, an observation that reconciles the occurrence of this mutation in the control population. These findings suggest that a destabilized form of PFN1 underlies PFN1-mediated ALS pathogenesis.

Overexpression of profilin reduces the migration of invasive breast cancer cells

The exact role profilin plays in cell migration is not clear. In this study, we have evaluated the effect of overexpression of profilin on the migration of breast cancer cells. Overexpression was carried out by stably expressing GFP-profilin in BT474 cells. It was observed that even a moderate level of overexpression of profilin significantly impaired the ability of BT474 cells to spread on fibronectin-coated substrate and migrate in response to EGF. GFP-profilin expressing cells also showed increased resistance to detachment in response to trypsin and increased tyrosine phosphorylation of focal adhesion kinase (FAK) and paxillin compared to the parental and GFP-expressing (control) cell lines. These results suggest that perturbation of profilin levels may offer a good strategy for controlling the metastatic potential of breast cancer cells.

Depolymerization of actin filaments by profilin. Effects of profilin on capping protein function

Profilin interacts with the barbed ends of actin filaments and is thought to facilitate in vivo actin polymerization. This conclusion is based primarily on in vitro kinetic experiments using relatively low concentrations of profilin (1-5 microm). However, the cell contains actin regulatory proteins with multiple profilin binding sites that potentially can attract millimolar concentrations of profilin to areas requiring rapid actin filament turnover. We have studied the effects of higher concentrations of profilin (10-100 microm) on actin monomer kinetics at the barbed end. Prior work indicated that profilin might augment actin filament depolymerization in this range of profilin concentration. At barbed-end saturating concentrations (final concentration, approximately 40 microm), profilin accelerated the off-rate of actin monomers by a factor of four to six. Comparable concentrations of latrunculin had no detectable effect on the depolymerization rate, indicating that profilin-mediated acceleration was independent of monomer sequestration. Furthermore, we have found that high concentrations of profilin can successfully compete with CapG for the barbed end and uncap actin filaments, and a simple equilibrium model of competitive binding could explain these effects. In contrast, neither gelsolin nor CapZ could be dissociated from actin filaments under the same conditions. These differences in the ability of profilin to dissociate capping proteins may explain earlier in vivo data showing selective depolymerization of actin filaments after microinjection of profilin. The finding that profilin can uncap actin filaments was not previously appreciated, and this newly discovered function may have important implications for filament elongation as well as depolymerization.

Evidence for two interaction regions for phosphatidylinositol(4,5)-bisphosphate on mammalian profilin I

The binding of phosphatidylinositol(4,5)-bisphosphate (PI(4,5)P(2)) to profilin at a region distinct from the actin interaction surface is demonstrated by experiments with covalently cross-linked profilin:beta-actin. The result is in agreement with observations made with several mutant profilins and provides strong evidence for two regions on mammalian profilin mediating electrostatic interaction with phosphatidylinositol lipids; one close to the binding site for poly(L-proline), and one partially overlapping with the actin-binding surface. Congruent with this, two plant profilins, which have a reduced number of positive amino acids in one of these regions, displayed a dramatically lower binding to PI(4,5)P(2) compared to human profilin I.

Effects of cross-linked profilin:beta/gamma-actin on the dynamics of the microfilament system in cultured cells

There is evidence that the profilin:actin complex is the immediate precursor in the formation of actin filaments in cells. This paper describes the cell morphology and microfilament distribution after microinjection of covalently cross-linked profilin:beta/gamma-actin (PxA) in two different cell lines. Injected cells were either kept unstimulated or stimulated with platelet-derived growth factor (PDGF) before fixation and visualization of filamentous actin. After injection of low doses of PxA, the cells displayed an actin organization characterized by a clearance of diffuse fluorescence from a region immediately interior of ruffling edges and the appearance of small dots of fluorescence in the same region. At higher concentrations, PxA effectively inhibited outgrowth of lamellae and microspikes, and there was a drastic reduction of actin staining in the zone behind the advancing edge. This effect is reminiscent of the effect of cytochalasin B on fibroblasts and the growth cone of neuronal cells. As in these cases, there remained a rim of actin-dependent fluorescence on the very edge of the membrane lamella, particularly in the PxA-treated fibroblasts. The interference of PxA with the formation of surface structures was pronounced after PDGF stimulation. Here, PxA effectively eliminated the enhancement of the ruffling activity in the cell edges and on the dorsal surface of the cells. In contrast to PxA, injection of non-cross-linked profilin:beta/gamma-actin had no apparent effect on cell morphology and microfilament distribution except for an increased concentration of filamentous actin in one of the cell lines.

Mammalian homolog of the yeast cyclase associated protein, CAP/Srv2p, regulates actin filament assembly

Control of cell shape and motility requires rearrangements of the actin cytoskeleton. One cytoskeletal protein that may regulate actin dynamics is CAP (cyclase associated protein; CAP/Srv2p; ASP-56). CAP was first isolated from yeast as an adenylyl cyclase associated protein required for RAS regulation of cAMP signaling. In addition, CAP also regulates the actin cytoskeleton primarily through an actin monomer binding activity. CAP homologs are found in many eukaryotes, including mammals where they also bind actin, but little is known about their biological function. We, therefore, designed experiments to address CAP1 regulation of the actin cytoskeleton. CAP1 localized to membrane ruffles and actin stress fibers in fixed cells of various types. To address localization in living cells, we constructed GFP-CAP1 fusion proteins and found that fusion proteins lacking the actin-binding region localized like the wild type protein. We also performed microinjection studies with affinity-purified anti-CAP1 antibodies in Swiss 3T3 fibroblasts and found that the antibodies attenuated serum stimulation of stress fibers. Finally, CAP1 purified from platelets through a monoclonal antibody affinity purification step stimulated the formation of stress fiber-like filaments when it was microinjected into serum-starved Swiss 3T3 cells. Taken together, these data suggest that CAP1 promotes assembly of the actin cytoskeleton.

Profilin is predominantly associated with monomeric actin in Acanthamoeba

We used biochemical fractionation, immunoassays and microscopy of live and fixed Acanthamoeba to determine how much profilin is bound to its known ligands: actin, membrane PIP(2), Arp2/3 complex and polyproline sequences. Virtually all profilin is soluble after gentle homogenization of cells. During gel filtration of extracts on Sephadex G75, approximately 60% of profilin chromatographs with monomeric actin, 40% is free and none voids with Arp2/3 complex or other large particles. Selective monoclonal antibodies confirm that most of the profilin is bound to actin: 65% in extract immunoadsorption assays and 74–89% by fluorescent antibody staining. Other than monomeric actin, no major profilin ligands are detected in crude extracts. Profilin-II labeled with rhodamine on cysteine at position 58 retains its affinity for actin, PIP(2) and poly-L-proline. When syringe-loaded into live cells, it distributes throughout the cytoplasm, is excluded from membrane-bounded organelles, and concentrates in lamellapodia and sites of endocytosis but not directly on the plasma membrane. Some profilin fluorescence appears punctate, but since no particulate profilin is detected biochemically, these spots may be soluble profilin between organelles that exclude profilin. The distribution of profilin in fixed human A431 cells is similar to that in amoebas. Our results show that the major pool of polymerizable actin monomers is complexed with profilin and spread throughout the cytoplasm.

Effects of single amino acid substitutions in the actin-binding site on the biological activity of bovine profilin I

For a detailed analysis of the profilin-actin interaction, we designed several point mutations in bovine profilin I by computer modeling. The recombinant proteins were analyzed in vitro for their actin-binding properties. Mutant proteins with a putatively higher affinity for actin were produced by attempting to introduce an additional bond to actin. However, these mutants displayed a lower affinity for actin than wild-type profilin, suggesting that additional putative bonds created this way cannot increase profilin's affinity for actin. In contrast, mutants designed to have a reduced affinity for actin by eliminating profilin-actin bonds displayed the desired properties in viscosity assays, while their binding sites for poly(L)proline were still intact. The profilin mutant F59A, with an affinity for actin reduced by one order of magnitude as compared to wild-type profilin, was analyzed further in cells. When microinjected into fibroblasts, F59A colocalized with the endogenous profilin and actin in ruffling areas, suggesting that profilins are targeted to and tethered at these sites by ligands other than actin. Profilin null cells of Dictyostelium were transfected with bovine wild-type profilin I and F59A. Bovine profilin I, although expressed to only approximately 10% of the endogenous profilin level determined for wild-type Dictyostelium, caused a substantial rescue of the defects observed in profilin null amoebae, as seen by measuring the growth of colony surface areas and the percentage of polynucleated cells. The mutant protein was much less effective. These results emphasize the highly conserved biological function of profilins with low sequence homology, and correlate specifically their actin-binding capacity with cell motility and proliferation.

The role of profilin in actin polymerization and nucleotide exchange

Properties of human profilin I mutated in the major actin-binding site were studied and compared with wild-type profilin using beta/gamma-actin as interaction partner. The mutants ranged in affinity, from those that only weakly affected polymerization of actin to one that bound actin more strongly than wild-type profilin. With profilins, whose sequestering activity was low, the concentration of free actin monomers observed at steady-state of polymerization [Afree], was close to that seen with actin alone ([Acc], critical concentration of polymerization). Profilin mutants binding actin with an intermediate affinity like wild-type profilin caused a lowering of [Afree] as compared to [Acc], indicating that actin monomers and profilin:actin complexes participate in polymer formation. With a mutant profilin, which bound actin more strongly than the wild-type protein, an efficient sequestration of actin was observed, and in this case, the [Afree] at steady state was again close to [Acc], suggesting that the mutant profilin:actin had a greatly lowered ability to incorporate actin subunits at the (+)-end. The results from the kinetic and steady-state experiments presented are consonant with the idea that profilin:actin complexes are directly incorporated at the (+)-end of actively polymerizing actin filaments, while they do not support the view that profilin facilitates polymer formation.

Isolation and characterization of two mutants of human profilin I that do not bind poly(L-proline)

A simple procedure for the isolation of profilin mutants having a reduced capacity to bind poly(L-proline) is used to isolate two mutants of human profilin I, W3N and H133S. Binding of the mutants to poly(L-proline), actin, and phosphatidylinositol (4,5)-bisphosphate (PIP2) was studied. Both mutations abolished the poly(L-proline)-binding activity of profilin. This suggests that the arrangement of the N- and C-terminal helices forming the poly(L-proline)-binding site depends on the stabilizing interaction between W3 and W31 in the underlying beta-strand, and that the H133S mutation in the C-terminal helix also must have distorted the arrangement of the terminal helices. Both mutations caused a reduced affinity for actin, with the W3N replacement having the most pronounced effect. This shows that structural changes in the poly(L-proline)-binding region of profilin can affect the distantly located actin-binding site. Thus, ligands influencing the structure of the poly(L-proline)-binding site may regulate actin polymerization through profilin. This is consonant with the finding that PIP2, which changes the tryptophan fluorescence in wild-type profilin and dissociates the profilin:actin complex in vitro, binds more strongly to the W3N mutant profilin. Thus, the poly(L-proline)-binding surface represents a crucial regulatory site of profilin function.